Add test coverage for database conversion and serialization methods

[Copilot]

Core vector classes (Vector, HalfVector, SparseVector, Bit) had untested database conversion methods (_to_db, _from_db, _to_db_binary, _from_db_binary) and serialization methods (to_text).

Changes

• Vector: Added 15 tests covering text serialization, database conversion with dimension validation, binary serialization, and null/ndarray handling
• HalfVector: Added 15 tests mirroring Vector coverage
• SparseVector: Added 13 tests for database conversion methods with proper type validation
• Bit: Added 8 tests for database conversion, binary serialization, and type validation

Coverage Impact

• Before: 58 tests
• After: 109 tests (+88%)

All new tests validate edge cases including null handling, dimension mismatches, type validation, and numpy array passthrough behavior.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve tests coverage in the repo</issue_title>
<issue_description></issue_description>

<agent_instructions>Follow the task, add only necessary tests not redundant ones</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve tests coverage in the repo #5

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