Preserve baselines for skipped tests

[mavam]

Summary

• stop treating existing baseline files as an error for skipped tests
• preserve skipped-test baselines during --update
• add regression tests for verbose, quiet, and update-mode skip handling

Testing

• uv run pytest tests/test_run.py -k 'handle_skip or run_skipped_reason_matches_static_skip_and_executes or suite_static_skip_short_circuits_fixture_activation or suite_static_skip_respects_run_skipped_selector'
• uv run ruff check src/tenzir_test/run.py tests/test_run.py
• uv run ruff format --check src/tenzir_test/run.py tests/test_run.py

Linear: https://linear.app/tenzir/issue/TNZ-438/dont-show-an-error-like-output-for-skipped-tests

_{resolves TNZ-438}

[greptile-apps]

Confidence Score: 4/5

• This PR is safe to merge; it removes a clearly incorrect baseline-file side-effect from skip handling with good test coverage.
• The logic change is small and well-targeted, the new and updated tests directly verify the intended behaviour, and the changelog entry is accurate. The only concerns are cosmetic/maintenance: two now-unused parameters on handle_skip and a stale bool | str return-type annotation.
• The update and output_ext parameters in handle_skip (src/tenzir_test/run.py line 4289) are now dead and worth cleaning up.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Worker._record_static_skip called] --> B[handle_skip invoked]
    B --> C{is_verbose_output?}
    C -- Yes --> D[Print SKIP message]
    C -- No --> E[No output]
    D --> F[return 'skipped']
    E --> F

    subgraph OLD ["Old behavior (removed)"]
        G[ref_path = test.with_suffix]
        H{update=True?}
        G --> H
        H -- Yes --> I[Write empty file to ref_path]
        H -- No --> J{ref_path exists and non-empty?}
        J -- Yes --> K[report_failure + return False]
        J -- No --> F
    end

    style OLD fill:#ffcccc,stroke:#cc0000,color:#000

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_{Last reviewed commit: 5ba2cfc}