Stop DataFusionToolset from materializing full results for a capped query

[ColtenOuO]

Summary

DataFusionToolset's query tool returns at most max_rows rows (default 50), but it ran the query unbounded and called to_pydict() on the whole result before slicing, so a broad query like SELECT * FROM big_table could exhaust worker memory. The query is now capped at max_rows + 1 rows with a DataFusion LIMIT before anything is materialized; the extra row only signals truncation. What the agent sees is unchanged apart from total_rows (see below).

Changes

• Run the query as session_context.sql(sql).limit(max_rows + 1).to_pydict() and stop reporting total_rows. Errors are wrapped in QueryExecutionException with the same message DataFusionEngine.execute_query produces, so the retry handling is unchanged.
• DataFusionEngine.execute_query gained a max_rows argument only in common-sql 1.34.0, while this provider supports >=1.33.0. Applying the limit on the DataFrame keeps the change working on 1.33.0 without bumping the dependency.
• Update the tests to mock the DataFrame chain and assert the limit(max_rows + 1) call.

Impact

Peak RSS of one query call on a local 3M-row parquet file (65 MiB), max_rows=50:

Query	Before	After	Time before → after
SELECT *	1121 MiB	219 MiB	4.20s → 0.05s
WHERE amount > 500	860 MiB	218 MiB	1.16s → 0.07s
ORDER BY amount DESC	1358 MiB	279 MiB	3.30s → 0.30s
GROUP BY (20 rows)	219 MiB	219 MiB	unchanged

Before, memory grew linearly with the result size; after, it no longer depends on the result's row count.

Trade-off

total_rows is no longer returned, since only max_rows + 1 rows are read, and knowing the total would require a full scan or a second COUNT(*) query. An untruncated result loses nothing because row_count is already the total. For a truncated result, an agent that needs the count can run COUNT(*), in line with the tool description and hint, which already tell it to aggregate in SQL rather than page through a truncated result. SQLToolset already omits total_rows whenever the driver can't report a trustworthy total.

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[ColtenOuO]

Resolve the conflict :D

[Lee-W]

one nit