chore: isolate project IPC response framing

[hatayama]

Summary

• Isolate project IPC Content-Length framing and response writes behind one stateless writer.
• Keep the bridge server focused on request and connection orchestration without changing emitted bytes.

User Impact

• There is no intended user-visible behavior change.
• CLI request acceptance, heartbeat delivery, final responses, cancellation, and connection handling remain unchanged.

Changes

• Move Content-Length frame construction, raw stream writes, and per-connection locked writes into UnityCliLoopBridgeResponseWriter.
• Keep ProcessRequestFrameAsync in the server and preserve its control flow.
• Update exactly three writer callers: the dispatch-accepted response, heartbeat callback, and final response.
• Keep the per-connection SemaphoreSlim, heartbeat lifecycle, disconnect monitor, request token source, and JSON generation in their existing owners.

Verification

• Passed the related EditMode baseline before editing (139/139).
• Confirmed the three new frame tests fail before implementation with three CS0103 errors, then pass after implementation (3/3).
• Covered empty output, exact ASCII framing, and UTF-8 multibyte byte length.
• Compared the three moved method bodies after normalizing required visibility/static changes; framing, guards, write parameters, lock handling, and try/finally behavior are unchanged.
• Passed the response writer, bridge shutdown, reassembler, heartbeat, cancellation policy, response serializer, client connection, architecture, and static facade guard fixtures (142/142).
• Compiled the Unity package with 0 errors and 0 warnings.

Compatibility

• The emitted frame remains Content-Length: <UTF-8 byte count>\r\n\r\n<JSON>.
• Dispatch-accepted, heartbeat, and final JSON shapes are still owned by JsonRpcRequestProcessor; their order is unchanged.
• No IPC contract changed, so the protocol version remains unchanged.

Follow-up

• R2-35 remains open. The next stage will move client stream/task ownership and per-client session processing into a dedicated session manager after separate plan review.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: b69d44bb-1d5c-4498-ad87-0d2df3c19524

📥 Commits

Reviewing files that changed from the base of the PR and between b47a227 and 1923431.

⛔ Files ignored due to path filters (2)

• Assets/Tests/Editor/UnityCliLoopBridgeResponseWriterTests.cs.meta is excluded by none and included by none
• Packages/src/Editor/Infrastructure/UnityCliLoopBridgeResponseWriter.cs.meta is excluded by none and included by none

📒 Files selected for processing (3)

• Assets/Tests/Editor/UnityCliLoopBridgeResponseWriterTests.cs
• Packages/src/Editor/Infrastructure/UnityCliLoopBridgeResponseWriter.cs
• Packages/src/Editor/Infrastructure/UnityCliLoopBridgeServer.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/hatayama/extract-bridge-response-writer

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