Account for TopK common-prefix converter memory

[kosiew]

Which issue does this PR close?

• Part of feat: analyze memory usage for size functions #23393

Rationale for this change

TopK underreports retained memory when a common sort prefix is present because its memory accounting omits the optional common_sort_prefix_converter. This can cause memory reservations to underestimate TopK's retained memory, weakening memory-limit enforcement.

What changes are included in this PR?

• Include common_sort_prefix_converter.size() exactly once in TopK::size() when the converter is present.
• Preserve the existing size calculation when the converter is absent.
• Add a regression test covering both construction paths and checking that the memory reservation matches the reported size after batch insertion.

Are these changes tested?

Added test_topk_size_common_sort_prefix_converter, which verifies:

• A common-prefix converter is created and reports a nonzero size.
• Without a common prefix, TopK::size() retains its existing formula.
• With a common prefix, the reported size increases by the converter's size.
• After insert_batch(), the memory reservation matches TopK::size().

The test does not rely on global allocator measurements.

Are there any user-facing changes?

No changes to query results or public APIs. The change corrects internal memory accounting for TopK with a common sort prefix, potentially improving memory-limit enforcement.

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.84%. Comparing base (0d79252) to head (316c2cd).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/physical-plan/src/topk/mod.rs	91.66%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26286      +/-   ##
==========================================
+ Coverage   82.80%   82.84%   +0.04%     
==========================================
  Files        1149     1151       +2     
  Lines      454146   455546    +1400     
  Branches   454146   455546    +1400     
==========================================
+ Hits       376035   377382    +1347     
+ Misses      55260    55246      -14     
- Partials    22851    22918      +67     

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[nuno-faria]

Thanks @kosiew, LGTM.

[nuno-faria]

Is this part necessary for this test?

[kosiew]

It’s not necessary for this regression. I added it to check memory-pool charging, but insert_batch() already resizes the reservation using self.size(), so this equality also passes with the converter omitted.
The preceding size-delta assertion catches the bug. I’ll remove the insertion and reservation assertion.

[kosiew]

🚀
@nuno-faria
Thank you for your review.