fix(net): backport FETCH requester FIN semantics

[kixelated]

Release's FETCH serving paths still treat a clean requester FIN as cancellation on draft 19, discarding standalone and joining fetch responses. Reuse the existing version-aware request-stream cancellation helper while waiting for a joining subscription and while buffering its response: pre-19 FIN still cancels; modern FIN only ends requester updates; resets and response STOP_SENDING still cancel.

Validation: the release draft-19 regression fails before the fix. The final matrix covers standalone/joining requests, FIN/reset, and every draft whose FETCH this release serves; it passes. Scoped just check passes.

Public API: none. Wire: existing draft cancellation semantics, no framing or version change. No package version bump or release. Main quest cleanup is tracked in #5231.

Combined release validation: a private tree applying all six implementation commits passes 1,469 moq-net library tests (four existing ignored tests), 252 JS decoder/publisher tests, and just test interop --all across Rust, Python, Go, JS/browser/native Node/native Bun, C, and GStreamer. The interop harness negotiates Lite; the IETF behavior is covered by the version-specific regressions.

(Written by GPT-6)

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T22:43:05.794929Z	a1cfe89	Draft marked ready

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[kixelated]

Approved to merge a1cfe89cc37da422179554062c3bc9489d728131 into release with a merge commit. CI is green, Codex gave a thumbs up, and no review threads remain.

This backports the existing FETCH requester FIN semantics: modern drafts continue serving after clean FIN while resets and response cancellation still cancel. Public API and wire framing are unchanged. Existing focused and combined-release validation covers the behavior. No additional changes or unresolved decisions.

(Written by GPT-6)