Own MCP HTTP response lifecycle

[lynnswap]

Purpose

Keep every MCP HTTP response source, heartbeat, physical write, and response-end acknowledgement owned until completion or shutdown.

Changes

• Extend each admitted RequestOperation with explicit response-end lifecycle state while keeping NIO as the request-order owner.
• Remove the unbounded tracked-stream producer and active-stream singleton.
• Run source consumption, heartbeat, and writer work as structured children of the registered request task.
• Use a one-body rendezvous so the source cannot read the next chunk until the previous physical write is acknowledged, with response bodies prioritized over heartbeats.
• Await non-stream response writes and physical close acknowledgements for non-keepalive and error responses.
• Remove the unsafe direct streaming seam and retain a validation-only test seam.

Testing

• Twelve focused response lifecycle tests
• Strict MCP and Host suites with warnings as errors
• swift test --build-system swiftbuild --no-parallel
• Compatibility consumer, API, and MCP gates
• ReviewMonitor xcodebuild test (15 tests)
• Independent concurrency and root scope/API/test audits

Screenshots

Not applicable.

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[lynnswap]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: 7791ee9b66

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