test: add persistence integration coverage

[beinan]

Summary

• add a python-side text persistence round-trip integration test
• cover optional image serialization by comparing stored bytes against Pillow output
• add a version checkout regression test to validate time-travel semantics

Testing

• python/.venv/bin/python -m pytest tests/test_persistence.py
• ~/.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