feat: Pluggable transport interface + NetHTTP default transport.

[sichanyoo]

Issue #, if available:
5196

Description of changes:

AI-generated summary of changes below

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smithy-ruby -- Pluggable Transport changes: what & why
Commit b5f2f092  "Pluggable transport interface + NetHTTP default transport."
Generated 2026-08-24 15:59:35 -0700
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 1. [MODIFIED] gems/smithy-client/lib/smithy-client.rb  (+3 / -1)
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    Adjusts the gem require order for the new transport stack: drops the requires for the
    deleted net_http/handler and the old transport.rb contract file, and adds net_http/stream,
    net_http/transport, and the generic send_handler (loaded after net_http/connection_pool so
    dependencies resolve in order).

 2. [MODIFIED] gems/smithy-client/lib/smithy-client/errors.rb  (+7 / -0)
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    Introduces Smithy::Client::NotSupportedError (it used to live in the now-deleted transport
    contract file). HTTP/1.1 streams raise it from #write / #close_write because a request
    body cannot be written after the request is transmitted -- that is HTTP/2 event-stream
    territory.

 3. [DELETED ] gems/smithy-client/lib/smithy-client/net_http/handler.rb  (+0 / -157)
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    Deletes the old push/callback Net::HTTP handler. Its job is replaced by the new split: a
    NetHTTP::Transport returning a pull-based Stream, plus a transport-agnostic SendHandler.
    This is the heart of the pluggable-transport refactor.

 4. [NEW     ] gems/smithy-client/lib/smithy-client/net_http/stream.rb  (+323 / -0)
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    New. The fiber-backed HTTP/1.1 stream implementing the pull contract: response_headers,
    each_chunk, write / close_write (raise NotSupportedError), and abort. A Fiber keeps
    Net::HTTP's block-scoped read_body context alive across the separate response_headers and
    each_chunk calls (that block also holds the socket's buffered bytes, including any body
    bytes over-read while parsing headers). Robustness added during review: (1) Guaranteed
    teardown -- if the consumer raises or the body isn't fully read, the connection is
    finished, not leaked. (2) Truncation detection -- Net::HTTP defaults ignore_eof: true and
    silently returns short Content-Length bodies, so the byte count is verified inside the
    fiber (right after read_body, still inside the pool session block) and a truncated,
    peer-closed socket is finished instead of returned to the pool. (3) Thread safety -- a
    mutex guards the @done/@aborted/@error/@session transitions so a cross-thread abort is
    observed and cannot race normal completion, and the fiber reconciles an abort that raced
    ahead of session checkout so the session is never orphaned. Note: the pull-based model
    holds on all runtimes, but the "no background OS thread" property is CRuby-only
    (JRuby/TruffleRuby back fibers with a thread).

 5. [NEW     ] gems/smithy-client/lib/smithy-client/net_http/transport.rb  (+154 / -0)
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    New. NetHTTP::Transport is the default HTTP/1.1 transport and the documented swap point
    (the :transport option). It exposes a single transmit(request) -> Stream, maps the
    transport-agnostic client options (connect/read timeouts, TLS verify + trust store, proxy,
    wire trace) onto the Net::HTTP ConnectionPool option names, and accepts Net::HTTP-specific
    knobs (continue/keep-alive/write/ssl timeouts, client cert/key) as constructor-only
    kwargs. The class doc carries a full V3-option equivalence table. close was intentionally
    omitted: with the global H1 pool it is not meaningful, and V3 has no H1 equivalent.

 6. [MODIFIED] gems/smithy-client/lib/smithy-client/net_http/connection_pool.rb  (+27 / -4)
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    Hardens TLS setup. configure_ssl now sets http.verify_mode explicitly so ssl_verify_peer:
    false is actually honored (previously it was silently ignored and always verified), and
    warns through the logger when verification is disabled. The old cert method is split into
    configure_client_cert (client cert/key for mutual TLS -- always applied, since it is
    independent of server verification) and configure_ca_trust (CA bundle/path/store --
    applied only when peer verification is on).

 7. [DELETED ] gems/smithy-client/lib/smithy-client/plugins/net_http.rb  (+0 / -163)
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    Deletes the old Net::HTTP plugin. Its options are re-homed: transport-agnostic ones move
    to the new generic Transport plugin; Net::HTTP-specific ones become constructor kwargs on
    NetHTTP::Transport (no longer client config).

 8. [NEW     ] gems/smithy-client/lib/smithy-client/plugins/transport.rb  (+172 / -0)
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    New. The generic, transport-neutral plugin. It defines the transport-agnostic client
    options (connect_timeout, read_timeout, ssl_verify_peer, ssl_ca_bundle/directory/store,
    http_proxy, http_wire_trace), resolves the :transport default (a NetHTTP::Transport built
    from those options, forwarded via a single TRANSPORT_OPTIONS list to avoid drift), and
    registers the SendHandler at the :send step. It validates a customer-supplied :transport
    in before_initialize (must respond to #transmit) -- chosen over after_initialize so the
    default transport isn't eagerly built just to validate it.

 9. [NEW     ] gems/smithy-client/lib/smithy-client/send_handler.rb  (+73 / -0)
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    New. The transport-agnostic :send handler. It calls transport.transmit(request), then
    bridges the pulled Stream bytes onto the push-based Http::Response (signal headers ->
    each_chunk -> signal_data -> signal_done). Blocking is a handler-stack decision, not a
    transport one: it resolves the response immediately for plain request/response but returns
    early for a bidirectional duplex_stream (owned by the future event-stream layer). An
    ensure guarantees the stream is aborted on any early/error exit so the connection is
    released.

10. [NEW     ] gems/smithy-client/spec/smithy-client/net_http/stream_spec.rb  (+100 / -0)
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    New. Unit coverage for the Stream: header/status reads, body chunk streaming, empty body,
    invalid-verb ArgumentError without networking, networking-error wrapping, write /
    close_write raising, and abort.

11. [NEW     ] gems/smithy-client/spec/smithy-client/net_http/transport_spec.rb  (+110 / -0)
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    New. Coverage for NetHTTP::Transport: transmit returns a Stream, the transport-agnostic ->
    Net::HTTP option mapping, and default wiring through the plugin.

12. [NEW     ] gems/smithy-client/spec/smithy-client/plugins/transport_spec.rb  (+56 / -0)
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    New. Coverage for the Transport plugin: tier-a options are defined, a default NetHTTP
    transport resolves, a customer-supplied transport is used as-is (and one lacking #transmit
    is rejected), option forwarding, and SendHandler registration.

13. [NEW     ] gems/smithy-client/spec/smithy-client/send_handler_spec.rb  (+126 / -0)
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    New. Coverage for the SendHandler: status/header/body population, invalid-verb and
    networking/OpenSSL error signaling, the Content-Length truncation raise, HEAD (no byte
    verification), and that a duplex_stream stores the stream without resolving the response.

14. [DELETED ] gems/smithy-client/spec/smithy-client/net_http/handler_spec.rb  (+0 / -174)
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    Deleted along with the old handler it tested.

15. [DELETED ] gems/smithy-client/spec/smithy-client/plugins/net_http_spec.rb  (+0 / -37)
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    Deleted along with the old Net::HTTP plugin it tested.

16. [MODIFIED] gems/smithy/lib/smithy/welds/default_plugins.rb  (+2 / -2)
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    Codegen weld change. Removes the old NetHTTP plugin from the default set and adds the new
    generic Transport plugin, positioned before StubResponses (both register a :send handler
    and the last one added wins, so stubbing must be able to override the real send handler).

17. [MODIFIED] projections/weather/lib/weather/client.rb  (+9 / -0)
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    Regenerated sample client showing the weld effect: the Transport plugin and :transport
    option are added (before StubResponses), replacing the old NetHTTP wiring.

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Totals: 17 files,  +1162 insertions,  -538 deletions
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