Add structured logging to MCP transport

[gearnode]

Summary

• adapt the application logger to the standard slog API used by the MCP SDK
• wire a named transport logger into the streamable HTTP handler
• preserve log levels, attributes, nested groups, and request contexts through the adapter
• cover the transport logging behavior with a focused unit test

Testing

• go test ./pkg/server/api/mcp/mcputils ./pkg/server/api/mcp/v1
• go vet ./pkg/server/api/mcp/mcputils ./pkg/server/api/mcp/v1

Linear Issue: ENG-772

  

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Summary by cubic

Adds structured logging to the MCP transport so SDK logs flow through our standard logger instead of being dropped. Old behavior: nil logger; new behavior: a named transport logger using slog preserves levels, attrs, and nested groups; no change to request handling. Addresses ENG-772.

MCP +117 -1

• Adds mcputils.NewSlogLogger, adapting go.gearno.de/kit/log to slog with WithAttrs and nested WithGroup.
• Wires logger.Named("transport") into the MCP v1 streamable HTTP handler.
• Preserves context, attributes, and group structure when forwarding records.

Tests +70 -0

• Adds a unit test that asserts level, message, logger name, attributes, and nested groups are preserved and encoded.
• Focuses the adapter test on MCP behavior and drops testing/slogtest usage.

^{Written for commit 1b6dbf7. Summary will update on new commits.}