[Data] Fuse MapBatches even if they modify the row count

[bveeramani]

This PR updates the operator fusion rule to fuse MapBatches even if they modify the row counts. The intention of this PR is to preserve the historical operator fusion behavior and avoid introducing regressions.

For more details, see the timeline below.

Timeline of Changes

Date	Event	Description
June 8, 2023	Limit pushdown added	Added limit pushdown and a property to MapBatches incorrectly stating it doesn't modify row counts. (#35950)
June 27, 2023	Limit pushdown disabled	Rule disabled because it incorrectly pushed limits past UDFs that modified row counts. (#36831)
April 28, 2025	Fusion restricted	Added logic to stop fusing operators that modify row counts when the downstream has a batch size. MapBatches stayed fused only because of its incorrect property (#52570).
July 8, 2025	Limit pushdown re-enabled with special case	Re-enabled with a special case to prevent pushing limits past MapBatches. (#39486)
Oct 24, 2025	Special case removed	Special case removed, re-introducing the bug where limits are pushed past MapBatches. (#57880)
Feb 2, 2026	Property Fix	Updated MapBatches to correctly report it modifies rows by default. This fixed the pushdown bug but broke fusion logic. (PR #60448)
Feb 4, 2026	(This PR)	Add a special-case to preserve the historical MapBatches fusion behavior

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[gemini-code-assist]

Code Review

This pull request introduces a special-case exception to allow MapBatches operators to be fused, even when they can modify row counts. This change preserves historical behavior and prevents performance regressions that were introduced when MapBatches was updated to correctly report its ability to modify row counts. The changes to the test files correctly reflect this new fusion logic by removing the now-unnecessary udf_modifying_row_count=False parameter. The overall change is well-justified and looks good. I have one minor suggestion to improve code maintainability.