fix: report LowerEqual cardinality effect for CoalescePartitionsExec with fetch

[asolimando]

Which issue does this PR close?

• Closes CoalescePartitionsExec::cardinality_effect returns Equal when fetch is set #26050.

Rationale for this change

CoalescePartitionsExec::cardinality_effect returns CardinalityEffect::Equal even when fetch is set, although the operator can then produce fewer rows than its input. Code that relies on cardinality_effect gets the wrong answer, for example PassthroughStatisticsProvider reports the input row count and drops the fetch.

What changes are included in this PR?

• CoalescePartitionsExec::cardinality_effect returns CardinalityEffect::LowerEqual when fetch is set, as SortExec and SortPreservingMergeExec already do.

What is the testing strategy for this PR?

• New unit test for cardinality_effect with and without fetch.
• New unit test showing that PassthroughStatisticsProvider no longer drops the fetch (Exact(10) instead of Exact(1000)).
• sqllogictest and the core physical_optimizer integration tests pass with no plan changes.

Are there any user-facing changes?

No.

---

Disclaimer: I used AI to assist in the code generation, I have manually reviewed the output and it matches my intention and understanding.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (c3ef346) to head (a170053).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...atafusion/physical-plan/src/coalesce_partitions.rs	83.33%	0 Missing and 2 partials ⚠️
...usion/physical-plan/src/operator_statistics/mod.rs	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26051   +/-   ##
=======================================
  Coverage   82.66%   82.66%           
=======================================
  Files        1147     1147           
  Lines      446357   446452   +95     
  Branches   446357   446452   +95     
=======================================
+ Hits       368971   369055   +84     
+ Misses      54997    54993    -4     
- Partials    22389    22404   +15     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[rgbuilds]

I traced the cardinality declaration through PassthroughStatisticsProvider and the operator statistics fallback. LowerEqual correctly prevents passthrough when fetch can reduce the output, allowing the existing fetch-aware calculation to cap the estimate.

The new tests cover both declaration branches and the resulting Exact(10) estimate from an Exact(1000) input. Restoring the previous Equal behavior makes the provider regression fail with Exact(1000). The focused physical-plan, optimizer, statistics, SQL logic, formatting, and Clippy checks pass locally. No blocking issues found.

[asolimando]

Hey @nuno-faria, thanks a lot for the approval! Is there anything pending or we can merge this?

[alamb]

I put it in the queue

Tanks @nuno-faria and @asolimando and @rgbuilds