test(core): expand unit coverage for core MCP modules

Part of #9. To build on the test scaffold in #6.

Raise core module coverage and clear TODO(test) markers under tests/unit/core/

Scope

- dynamic_tools: registry, progressive/semantic search, keyword matching
- server: create_server, persona filtering, read-only mode, tool registration
- health and logging: health check, formatters, correlation IDs
- resources: URI resolution and MCP resource registration
- auth: JWT validation paths not yet covered
- tool_integrity: pin description checksum baseline
- config: malformed YAML, auth/OTEL env overrides

Out of scope

- Trainer tools (separate issue)
- Integration/e2e
- Benchmarks (#10 / #26)

Done when
- TODO(test) markers in tests/unit/core/ cleared or tracked
- make test-python green
- Meaningful coverage increase for kubeflow_mcp/core/