fix(threads): collect nested replies from the conversation edge

[kevin9327]

Summary

ThreadsAdapter.ListComments read GET /{media-id}/replies, which returns only the top-level replies to a post. Any reply to a reply never reached the engagement inbox. That includes the most common follow-up, a follower answering the account's own reply. This change reads GET /{media-id}/conversation instead, which returns replies at every depth, and sets ParentID from replied_to so the inbox threads each reply under the one it answers.

Motivation

From the Threads Replies and Conversations reference:

• /replies "only returns the top-level replies under the Threads ID provided in the request".
• /conversation "returns all replies, regardless of the depth, either in chronological or reverse chronological order". Its reverse parameter defaults to true (newest first), the same as /replies.
• Both edges take the same fields, including is_reply_owned_by_me and hide_status, which the adapter already reads. They need the same permission, since the reference's permission list scopes it to GET calls on reply endpoints.
• replied_to is "Media ID of the immediate parent of the reply" and is returned as {"id": ...}.

This is the same gap #257 closed for PieFed's reply tree.

Threading follows what the other adapters already do (#240). A nested reply's ParentID is the ID of the reply it answers, which the inbox matches against remote_id. A top-level reply's replied_to is the post itself, which is not a collected comment, so it still shows as top level. The account's own replies keep IsOurs from is_reply_owned_by_me, so they stay out of the inbox as before.

Testing

• Backend tests pass (go test ./internal/platform/...)
• Frontend tests pass (bun run --filter @openpost/web test): no frontend change
• Linting passes (go vet, gofmt, gofumpt, golangci-lint run ./internal/platform/...: 0 issues, same as main)
• Manually tested:
  • New TestThreadsListCommentsCollectsNestedReplies serves both edges the way the reference describes them. /replies holds only the top-level reply, and /conversation holds it plus the account's own reply and a follower's answer to that reply. The test checks that all three are collected, that the follow-up is threaded under the account's reply, and that ownership is kept.
  • The existing TestThreadsListCommentsMarksOwnRepliesAsOurs now expects the /conversation path.

Fails on unmodified main:

=== RUN   TestThreadsListCommentsCollectsNestedReplies
    threads_comments_test.go:68:
        	Error:      	"map[reply-fan:{reply-fan    fan   Does it support video? ...}]" should have 3 item(s), but has 1
--- FAIL: TestThreadsListCommentsCollectsNestedReplies (0.00s)

Passes with the fix. go test ./internal/platform/... is ok, and go build -tags dev ./... is clean. go test -coverprofile shows every changed line runs under the tests. I have not tested this against a live Threads account. The behavior is taken from the reference quoted above.

Checklist

• My code follows the project's coding conventions
• I have added tests that prove my fix is effective
• I have updated documentation if needed: added changes/threads-nested-replies.md
• I have used Conventional Commits for my commit messages
• I have checked that there are no other PRs open for the same issue/feature

Breaking Changes

• Yes
• No

Affected Components

• Backend (Go)

Platform Impact

• Threads

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📥 Commits

Reviewing files that changed from the base of the PR and between 542f87b and fbbdec0.

📒 Files selected for processing (3)

• apps/server/internal/platform/threads.go
• apps/server/internal/platform/threads_comments_test.go
• changes/threads-nested-replies.md

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