fix: preserve fetched merges during distribution enforcement

[discord9]

Which issue does this PR close?

Related to #14150 and #23800. This is independently reproducible on main and does not depend on #23800.

Rationale for this change

Running the default physical optimizer a second time can change query results. On main 8a9228164fddeaa03c9ee510b641990e403f2eb5, a global LIMIT over a partitioned aggregate returns one row after the first optimization and three after the second: distribution enforcement removes the CoalescePartitionsExec(fetch=1) that limits the combined output.

Fetched sort-preserving merges have an additional requirement: their ordering determines which rows survive. Replacing SortPreservingMergeExec(fetch=N) with CoalescePartitionsExec(fetch=N) can preserve the row count while selecting the wrong rows, even when the parent does not require ordered output.

What changes are included in this PR?

• Stop stripping distribution operators at the first operator carrying a fetch, including fetch=0. Preserve the actual global row-selection boundary rather than moving its numeric fetch elsewhere.
• Stop order-preserving-variant downgrades at fetched sort-preserving merges before recursing into their ordered inputs.
• Remove the now-unreachable collection and reapplication of fetch from stripped operators. Fetch-free distribution optimization remains unchanged.
• Correct the existing single-partition fetched-SPM snapshot to retain a local limit and satisfy the merge's input ordering.

What is the testing strategy for this PR?

Native execution regressions cover:

• One- and three-partition fetched coalesces, including zero fetch.
• LIMIT over a partitioned aggregate through two complete default optimizer passes.
• TopK beneath a genuinely order-insensitive aggregate, asserting selected values rather than merely row counts.
• OFFSET + LIMIT and OFFSET-only, including exact ordered output values.
• A genuine per-partition LocalLimit that must still return one row from each partition.
• Fetched and fetch-free SPM downgrade behavior, including order-preserving repartition descendants.

Baseline execution on unmodified main (8a9228164f) and the latest #23800 head (a403530d99) demonstrated the global LIMIT regression through two complete default physical optimizer passes: one row on the first pass, three on the second.

Independent guard-removal experiments on #23800 also verified both protections:

Variant	Executed result
Original production code	TopK returns [10,20,98,99] instead of [10,20].
Remove the fetched-node removal guard	LIMIT returns one row after the first full pass and three after the second.
Remove the fetched-SPM downgrade guard	Both passes return two rows, but select [10,99] instead of [10,20].
Both guards present	All 87 distribution tests pass.

On the main-based branch, the following checks pass:

cargo fmt --all --check
cargo test -p datafusion --test core_integration
# 1125 passed
cargo test -p datafusion-physical-optimizer --lib
# 37 passed
cargo clippy --all-targets --all-features -- -D warnings
git diff --check

The fix combined with #23800 additionally passes 87 distribution tests, 42 LimitPushdown tests, 30 EnsureRequirements tests, and strict all-target/all-feature Clippy for datafusion and datafusion-physical-optimizer.

Are there any user-facing changes?

Queries retain their global LIMIT/OFFSET and ordered TopK selection when physical plans are optimized again. No public API or configuration changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.74%. Comparing base (8a92281) to head (b50c877).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25065      +/-   ##
==========================================
+ Coverage   81.72%   81.74%   +0.02%     
==========================================
  Files        1127     1128       +1     
  Lines      416519   416633     +114     
  Branches   416519   416633     +114     
==========================================
+ Hits       340401   340587     +186     
+ Misses      56115    55988     -127     
- Partials    20003    20058      +55     

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