test: add missing model and error code test coverage

[julianschiavo]

• Add missing .invalidScore to BridgeErrorCode exhaustive test
• Add tests for unknown DocumentationKind decoding and NaN/infinity score encoding
• Fix DocumentationEntry to decode unrecognized kind values as nil instead of throwing

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Note

Low Risk
Low risk: behavior change is limited to DocumentationEntry JSON decoding, making it more tolerant of unexpected kind values; remaining changes are test-only and validate encoding failures for invalid floating-point scores.

Overview
Makes model decoding more robust and expands edge-case test coverage. DocumentationEntry now implements a custom Decodable initializer that treats unrecognized DocumentationKind raw values as nil instead of failing decode.

Tests are extended to include the previously-missing .invalidScore BridgeErrorCode, verify unknown kind decoding behavior, and assert that SearchResult with NaN/infinity scores throws during JSONEncoder encoding.

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