fix(docs-mcp): hide internal exception details from HTTP clients

[thecompez]

Summary

• Replace public 500 responses containing exception messages with a stable {error: internal_error} envelope.
• Log a structured diagnostic event with only the error class, not the potentially sensitive message.
• Keep the change in the single source server.mjs file to preserve simple deployment setups.

Evidence

• The original live error response disclosed the detail Invalid URL for a malformed Host request.
• The equivalent patched server was tested in isolation; the same malformed request returned only {error: internal_error}, while health and MCP requests still succeeded.
• node --check and git diff --check succeeded.
• No production restart or deploy. Live on-disk server was already patched defensively, but running Node process still uses its previous loaded code until controlled restart.

Safety

This is independent of unshipped rate-limiter changes and does not alter healthy request routing.