fix(mcp): log the server/discover connect probe as info, not an error

[mvanhorn]

What does this PR do?

That call now passes false for the error argument when the method is server/discover and true for every other unknown method, so the probe is logged as level=info with status=ok while other misses stay level=warn and status=error. The JSON-RPC response is still -32601 Method not found. The new test server_handle_discover_probe_logs_info checks those log fields, checks that unknown/method still logs a warning, checks that an initialize request for 2026-07-28 still negotiates 2025-11-25, and checks that resources/list still returns an empty resource list.

Every connect from a current MCP client sends server/discover first. This server answers with JSON-RPC -32601 Method not found, and the daemon records that reply as a warning with status=error, so each successful session leaves one recurring error line even though initialize and the later calls succeed. The method-not-found path in cbm_mcp_server_handle always called cbm_log_mcp_request with the error argument set to true, which is written as level=warn and status=error. server/discover is not implemented, so the modern connect probe takes that path and gets the same log treatment as any other unknown method. The -32601 body is the fallback signal a legacy server is supposed to return so the client continues with initialize.

Fixes #2433

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
  Not claimed: ran ./scripts/test.sh locally; it hit its time limit before finishing.
• Lint passes (make -f Makefile.cbm lint-ci)
  Not claimed: make -f Makefile.cbm lint-ci was not run locally either.
• New behavior is covered by a test (reproduce-first for bug fixes)

AI was used for assistance.

[github-actions]

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Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

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