feat: add context search API

[beinan]

Summary

• add Lance-backed nearest neighbor search to the Rust ContextStore and project results into typed records
• expose context.search through the Python bindings with structured dict payloads
• add Python helpers/tests for query coercion and result normalization

Testing

• python/.venv/bin/python -m pytest
• ~/.cargo/bin/cargo test --manifest-path rust/lance-context/Cargo.toml
• ~/.cargo/bin/cargo fmt --manifest-path rust/lance-context/Cargo.toml -- --check
• ~/.cargo/bin/cargo clippy --manifest-path rust/lance-context/Cargo.toml --all-targets -- -D warnings
• python/.venv/bin/ruff format --check python
• python/.venv/bin/ruff check python
• python/.venv/bin/pyright