test: add validator decision contract coverage

[blackdragoon26]

Hi maintainers,

This adds a focused, dependency-light contract suite for the example validator and its CI decision policy.

The change covers:

• inventory filtering, target selection, empty selections, and POSIX/Windows path normalization;
• PASS, FAIL, ERROR, WARNING, and SKIP aggregation, including executed-versus-skipped semantics;
• base/head classification for new, pre-existing, fixed, warning-only, and skipped results;
• malformed, missing, legacy, and incompatible result artifacts through a versioned result contract;
• explicit mock-runtime labeling and base/head workflow artifact isolation; and
• a normal pull-request workflow that runs without external APIs, model downloads, datasets, GPUs, or specialized hardware.

Verification:

python -m unittest discover -s .github/workflows/validator/tests -p 'test_*.py' -v
Ran 28 tests: OK

python -m compileall -q .github/workflows/validator

I also exercised static result generation, report aggregation, and identical base/head regression comparison end to end. The generated artifact used schema version 1, recorded execution, and remained non-blocking for an unchanged comparison.

Closes #838

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[blackdragoon26]

Pinging for the review, thanks.
@hsj576 @MooreZheng