feat(python): provide sql API through NamespaceSession

[majin1102]

Close #5781

[github-actions]

Code Review

P0: Breaking change to default features

The change to python/Cargo.toml modifies the default features:

[features]
-default = []
+default = ["rest", "rest-adapter"]

This is a breaking change that will force all Python builds to include REST dependencies by default. This should either:

1. Be reverted to keep default = [] (recommended)
2. Be documented as an intentional breaking change with justification

If the intent is to make REST features available for the PyNamespaceSessionBuilder, consider making this opt-in via a separate feature flag or documenting why these features are required by default.

P1: Incomplete docstrings in Python bindings

The NamespaceSession class and PyNamespaceSessionBuilder methods would benefit from docstrings on the Rust side (via #[pyo3(text_signature = "...")]) or more complete Python documentation, especially for the config parameter supported keys.

Minor observations (not blocking)

• The test coverage is good with parameterized join scenarios, aggregations, CTEs, and error cases
• The sql() method returns a combined pyarrow.Table which is a reasonable design choice
• The to_lance_namespace helper function correctly handles both low-level Rust types and high-level Python wrappers

Overall the implementation looks correct. The main concern is the default features change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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