fix(py): custom reader tests for new execution pipeline

[cpsievert]

This PR fixes 4 failing Python tests (for custom readers) that broke after #82 (7a5ed62).

These failures weren't caught at the time since the Python tests only run when there are changes to the Python package.

I'd be open to running the Python tests on every code change, but I'm not sure that's noise that the rest of the team wants to adopt?

Context

PR #82 ("New Scale syntax and implementation") introduced a column renaming pipeline where the execution engine generates SQL like:

SELECT "x" AS "__ggsql_aes_x__", "y" AS "__ggsql_aes_y__" FROM (...) AS __ggsql_src__

This SQL is passed to readers via execute_sql(), and the returned DataFrame is expected to contain the prefixed __ggsql_aes_* columns for downstream processing (pruning, scale resolution, etc.).

The custom reader tests were written with static readers that returned hardcoded DataFrames, ignoring the sql parameter:

class SimpleReader:
    def execute_sql(self, sql: str) -> pl.DataFrame:
        return pl.DataFrame({"x": [1, 2, 3], "y": [10, 20, 30]})

This worked with the old pipeline (which didn't rename columns via SQL), but the new pipeline expects the reader to execute the SQL it receives.

These failures were never caught on main because the Python CI workflow (python.yml) only triggers on changes to ggsql-python/**, and #82 only modified src/ and tree-sitter-ggsql/ files.

Changes

Test fixes: Updated 4 custom reader tests to use in-memory duckdb.connect() instances that actually execute the SQL they receive. This is more representative of real custom readers (like the existing IbisReader test) and properly exercises the bridge path.

Test dependencies: Added duckdb and pyarrow to both [project.optional-dependencies] test and [dependency-groups] dev. The pyarrow dependency is needed for duckdb's .pl() method to convert results to Polars DataFrames.

CI workflow fix: Changed pip install --find-links target/wheels/ ggsql[test] to pip install target/wheels/ggsql-*.whl'[test]'. This ensures that the locally built wheel will be installed and used, even if there is some more recent version available somewhere else.

🤖 Generated with Claude Code

[cpsievert]

Merging this without review since the only changes are Python tests and It fixes a problem already on main (several other PRs want this change to get free check marks).