fix: PartitionedTopKRank counts tie rows twice in output_rows

[jayzhan211]

Which issue does this PR close?

• N/A — no issue filed; found while reading the code.

Rationale for this change

For a RANK() window Top-N (PartitionedTopKExec: fn=rank), the output_rows and output_batches metrics shown by EXPLAIN ANALYZE are too high whenever rows tie at the K-th value.

PartitionedTopKRank::emit calls record_output on every tie batch before pushing it into the BatchCoalescer, and then calls record_output again on each completed batch coming out of that coalescer — which already contains those tie rows. Heap rows are only counted at the second site, so each tie row is counted twice and each tie batch adds a phantom output batch.

Example: K = 2, one partition with values 5, 5, 10, 5. Three rows are emitted in one batch, but the metrics report output_rows=4, output_batches=2.

PartitionedTopK (ROW_NUMBER) and PartitionedTopKDenseRank only record at the coalescer output and are not affected.

What changes are included in this PR?

Remove the extra record_output call on tie batches so every emitted row is counted once, at the coalescer output, as in the other two operators. Query results are unchanged; only the metrics are corrected.

What is the testing strategy for this PR?

New unit test test_partitioned_topk_rank_output_rows_counts_ties_once, which emits heap rows plus a boundary tie and asserts output_rows / output_batches equal what the stream actually produced. It fails on main with (2, 4) vs (1, 3).

Are there any user-facing changes?

output_rows / output_batches reported for PartitionedTopKExec with fn=rank are now accurate when ties are present. No API changes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.86207% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.38%. Comparing base (d20936c) to head (de09948).

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/topk/mod.rs	75.86%	1 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25535      +/-   ##
==========================================
- Coverage   82.38%   82.38%   -0.01%     
==========================================
  Files        1138     1138              
  Lines      434491   434519      +28     
  Branches   434491   434519      +28     
==========================================
+ Hits       357969   357981      +12     
- Misses      54875    54884       +9     
- Partials    21647    21654       +7     

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