feat: add metadata filters for context retrieval

[dcfocus]

Summary

• Add structured JSON metadata support for context records.
• Add list/search filters for built-in fields, timestamp ranges, and metadata equality/contains.
• Expose Python metadata and filters APIs with docs and tests.

Testing

• cargo fmt --all -- --check
• cargo check -p lance-context-core
• cargo check -p lance-context-python
• cargo test -p lance-context-core
• cargo clippy -p lance-context-core -p lance-context-python --all-targets -- -D warnings
• ruff format --check
• ruff check
• pyright
• python/.venv/bin/pytest python/tests/test_search.py python/tests/test_persistence.py::test_metadata_and_filters_round_trip python/tests/test_persistence.py::test_search_applies_filters_before_limit

Closes #53