test(trainer): expand unit coverage for trainer MCP tools

Part of #9. Builds on the test scaffold in #6.
Cover trainer tool success and submission paths using mock_trainer_client and mock_k8s_apis

Scope

- training: confirmed=True paths for fine_tune, run_custom_training, run_container_training
- discovery: list/get jobs success paths, runtime listing edge cases
- monitoring: log/event parsing, failure-pattern hints
- lifecycle: suspend/resume/delete with mocked SDK
- platform: runtime CRUD preview and confirmed paths
- planning: pre_flight and estimate_resources with mocked cluster data
- common/utils: is_mcp_managed with mocked CustomObjects API

Out of scope

- Real cluster calls
- Integration/e2e
- Benchmarks (#10 / #26)

Done when

- TODO(test) markers in tests/unit/trainer/ and related common/ cleared or tracked
- Pattern C tests (mock_trainer_client + verify_tool_*) for all write tools
- make test-python green