test: improve patch coverage for _validators.py and retrievers.py

[loookashow]

• Add tests/test_validators.py with direct tests for StrictHybridConfig,
  StrictVectorBoostConfig, validate_search_kwargs, and split_search_kwargs
• Add text mode tests for offset and sort passthrough (sync + async)
• Add async boosted mode test without custom embeddings
• _validators.py: 84.61% → 100%, retrievers.py: 95.65% → 97%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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