feat: add context checkout by version

[beinan]

Summary

• expose ContextStore::checkout/version so Python can access consistent dataset versions
• update Python bindings and API to accept numeric version IDs for checkout
• extend tests to cover the new behavior

Testing

• cargo fmt --manifest-path rust/lance-context/Cargo.toml -- --check
• cargo test --manifest-path rust/lance-context/Cargo.toml --lib
• uv run pytest
• uv run ruff format --check python/
• uv run ruff check python/
• uv run pyright