fix: reliability first timeout recovery

[lynnswap]

Summary:

• Normalize timeout/error responses to MCP-compatible JSON-RPC errors (HTTP 200) for both JSON and SSE paths, avoiding text/plain transport breakage.
• Refactor HTTP handling into dedicated internal components (HTTPRequestValidator, MCPMethodDispatcher, MCPResponseEmitter, MCPErrorResponder) and keep HTTPHandler focused on orchestration.
• Add upstream health states (healthy/degraded/quarantined) with timeout-driven quarantine, pin clearing, and probe-based recovery; return -32001 upstream unavailable when no healthy upstream is available.
• Make upstream writes non-blocking via queued async writer path with queue limits and fail-fast -32002 upstream overloaded handling.
• Extend/adjust tests for timeout mapping cleanup, batch timeout accounting, quarantine behavior, overloaded initialize/request flows, and non-blocking write responsiveness.

Testing:

• swift test --filter XcodeMCPProxyTests --parallel

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