fix(mcp-server): add diagnostic logging for silent HTTP 500 from transport

[Scra3]

Summary

• Capture and log the HTTP response body when the transport returns status >= 500
• Log after handleRequest when the transport returned a 500

Context

A client gets a silent 500 on notifications/initialized after a successful initialize. The server logs show:

info: [MCP] Incoming POST /mcp [initialize]
info: [MCP] [200] POST /mcp - 7ms
info: [MCP] Incoming POST /mcp [notifications/initialized]
error: [MCP] [500] POST /mcp - 1ms

No error message is logged between the "Incoming" and the "[500]".

This happens because the Hono adapter (@hono/node-server) inside @modelcontextprotocol/sdk catches errors internally and writes 500 directly to the Node.js response, bypassing our logger entirely. Two error paths in Hono produce silent 500s:

1. handleFetchError — when the Web Standard transport handler's Promise rejects → returns Response(null, {status: 500}) (empty body)
2. handleResponseError — when writing the response to the Node.js res fails → writes Error: <message> as plain text body

With this PR, the same scenario would log:

info: [MCP] Incoming POST /mcp [notifications/initialized]
error: [MCP] HTTP 500 response body: Error: <actual error message>    ← NEW
error: [MCP] Transport returned HTTP 500 for [notifications/initialized]  ← NEW
error: [MCP] [500] POST /mcp - 1ms

Test plan

• All 487 existing MCP server tests pass
• Lint passes
• Deploy to client environment and check logs for the actual error message

🤖 Generated with Claude Code

[qltysh]

Coverage Impact

⬆️ Merging this pull request will increase total coverage on main by 0.05%.

Modified Files with Diff Coverage (2)

Rating	File	% Diff	Uncovered Line #s
	packages/mcp-server/src/server.ts	100.0%
	packages/mcp-server/src/utils/sse-error-logger.ts	100.0%
	Total	100.0%

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