refactor(voice): extract ConversationRelay TwiML logic into twiml.py

[xinghaohuang91]

Summary

• VoiceChannel owned every TwiML layering/resolution decision inline (_build_twiml_options, _overlay_fields, _resolve_action_url, _resolve_websocket_url, _resolve_default_action_url), coupling the channel to ConversationRelay's TwiML shape.
• Moves all of it into a new TwiMLBuilderConversationRelay in twiml.py, constructed from TACConfig + VoiceChannelConfig, with a single build() entry point.
• VoiceChannel now just constructs the builder in __init__ and calls build() at its two TwiML call sites (handle_incoming_call, initiate_outbound_conversation).
• Pure move — no logic changes, no public API changes. Method bodies are unchanged apart from the necessary renames (self.tac.config → self.tac_config, self.config → self.channel_config).
• This is groundwork for a follow-up PR that splits ConversationRelay support out of VoiceChannel into a pluggable provider — this extraction keeps that PR's diff small by giving it a self-contained unit to move, instead of moving tangled logic and standing up the new abstraction in the same change.

Known minor, low-risk behavior nuance (documented, not fixed here): in initiate_outbound_conversation, TwiML rendering now happens just before the try block instead of inside it, so an exception from TwiML rendering itself (very unlikely — fields are already validated by this point) would skip the "Failed to initiate outbound call" log line but still propagate the same way.

Test plan

• make lint / make type-check / make check all pass
• 876 existing tests pass unmodified
• tests/ and getting_started/ have zero diff — public API and examples untouched

[Copilot]

Pull request overview

Refactors the Voice channel’s ConversationRelay TwiML composition so VoiceChannel no longer owns option layering and URL resolution, by extracting that logic into a dedicated builder in twiml.py. This reduces coupling between the channel implementation and the ConversationRelay TwiML shape, setting up future provider/plugin work without changing behavior.

Changes:

• Introduced TwiMLBuilderConversationRelay (in twiml.py) to encapsulate TwiML option layering and action/WebSocket URL resolution behind a single build() entry point.
• Updated VoiceChannel to construct and use the builder at its inbound/outbound TwiML call sites.
• Updated VoiceChannelConfig.default_twiml_options documentation to point at the new overlay logic location.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/tac/channels/voice/twiml.py	Adds TwiMLBuilderConversationRelay to centralize TwiML layering + URL resolution and reuse existing generate_twiml().
src/tac/channels/voice/channel.py	Replaces inline TwiML layering/resolution with builder usage for inbound and outbound TwiML generation.
src/tac/channels/voice/config.py	Updates the default_twiml_options doc reference to the extracted overlay implementation.

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