Codify the shared response-writer wrapper pattern

[Copilot]

The semantic clustering audit identified internal/httputil/BaseResponseWriter and internal/server/responseWriter as a near-duplicate pair, but the current structure is intentional: server.responseWriter already extends the shared status-capturing wrapper rather than reimplementing it. This change makes that relationship explicit and adds coverage for the contract that matters across wrappers.

• Pattern clarification

  • Tightens the BaseResponseWriter doc comment to state that it is the shared embed-first wrapper for package-specific response writers.
  • Clarifies that the server wrapper adds body capture/debug behavior on top of shared status capture and Unwrap passthrough.

• Contract coverage

  • Adds server-level coverage for implicit 200 OK status capture on first Write.
  • Adds coverage that the embedded wrapper still exposes optional http.ResponseWriter interfaces through http.ResponseController, preserving the intended Unwrap behavior for derived wrappers.

• Example

  type responseWriter struct {
      httputil.BaseResponseWriter
      body bytes.Buffer
  }
  
  rc := http.NewResponseController(w)
  err := rc.Flush() // succeeds when the underlying writer implements http.Flusher

This keeps the existing package organization intact while making the shared-wrapper pattern easier to recognize and less likely to be flagged as accidental duplication in future audits.

• Fixes [refactor] Semantic Function Clustering Analysis — response_writer duplication and package organization #10584

[Copilot]

Pull request overview

Clarifies the shared response-writer embedding pattern identified in #10584 and strengthens its contract coverage.

Changes:

• Documents BaseResponseWriter as the shared wrapper base.
• Verifies implicit 200 OK capture.
• Verifies optional-interface passthrough through Unwrap.

Show a summary per file

File	Description
internal/httputil/response_writer.go	Clarifies shared wrapper usage.
internal/server/response_writer.go	Documents inherited interface passthrough.
internal/server/response_writer_test.go	Adds status and http.Flusher coverage.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced

[github-actions]

🔒 mcpg Read-Only Stress — default AWF

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: default AWF (normal container isolation)

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (issues/PRs/file/commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	Error [-32602]: unknown tool	BLOCKED	✅
C	CLI	reads (github MCP bridge)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	Permission denied (gh unauthenticated)	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	Permission denied (gh unauthenticated)	BLOCKED	✅

Overall: PASS

Note on Part B: The gateway enforces read-only by removing write tools entirely from the MCP tool list. Write attempts return Error [-32602]: unknown tool — gateway-level enforcement (tool absent from allowed set).

Note on Parts D & E: gh CLI has no token in this runtime, so all REST and GraphQL write attempts fail with Permission denied. Effective boundary: mcpg (read-only MCP) + unauthenticated gh.

References: §30772710499

🔒 mcpg read-only stress (default AWF runtime) by Read-Only Stress: default runtime

[github-actions]

🔒 mcpg Read-Only Stress — gVisor

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: gVisor (runsc) kernel-level isolation

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (list_issues/list_prs/get_file/list_commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	unknown tool (gateway strips write tools)	BLOCKED	✅
C	CLI	reads (list_issues/get_file via github CLI)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	gh unauthenticated — exit 4	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	gh unauthenticated — exit 4	BLOCKED	✅

Overall: PASS

Part B refusals are Error [-32602]: unknown tool "<write_tool>" — gateway enforces read-only by stripping all write tools from the backend tool list. Parts D and E are blocked because gh has no token in this sandbox.

Run: §30772710508

🔒 mcpg read-only stress (gVisor runtime) by Read-Only Stress: gVisor runtime

[github-actions]

🔒 mcpg Read-Only Stress — docker-sbx

Surface coverage: MCP tool calls + proxied CLI (REST) + GraphQL mutations
Isolation runtime: docker-sbx (KVM-isolated microVM)

Part	Surface	Op	Result	Expected	Status
A	MCP	reads (issues/PRs/file/commits)	data returned	ALLOWED	✅
B	MCP	writes (reaction/star/issue/comment/branch/file/PR)	unknown tool [-32602]	BLOCKED	✅
C	CLI	reads (github CLI)	data returned	ALLOWED	✅
D	CLI	REST writes (reaction/star/issue/comment/file)	gh not authenticated	BLOCKED	✅
E	CLI	GraphQL mutations (addReaction/addStar/createIssue)	gh not authenticated	BLOCKED	✅

Overall: PASS

Note on Part B: Write tools absent from gateway registry (MCP -32602: unknown tool). gh CLI unauthenticated, adding enforcement for Parts D/E.

References: §30772710502

🔒 mcpg read-only stress (docker-sbx runtime) by Read-Only Stress: docker-sbx runtime