fix: relax Python Lance runtime dependencies

[dcfocus]

Summary

• Move Python Lance packages out of base runtime dependencies into a new lance-python extra.
• Keep pyarrow>=14 as the base Python dependency for dataframe serialization paths.
• Keep Python Lance packages in tests and install the wheel with [lance-python] in Python CI so raw dataset inspection coverage remains active.
• Document the new extra and add a packaging metadata regression test.

Closes #67

Testing

• uv lock --check
• python3 -m py_compile python/python/lance_context/api.py python/python/lance_context/__init__.py
• uv run --project python --extra dev ruff format --check python/
• uv run --project python --extra dev ruff check python/
• cd python && uv run --extra dev pyright
• uv run --project python --extra tests pytest python/tests/test_packaging_metadata.py python/tests/test_search.py -q
• git diff --check

Not run locally:

• Full CI helper / Rust checks: local cargo is not installed in this environment.