fix: preserve fetch across distribution reoptimization

[xudong963]

Which issue does this PR close?

• Closes EnsureRequirements can silently drop fetch during distribution reoptimization #24807.

Rationale for this change

Reoptimizing an already optimized physical plan could silently remove a pushed-down LIMIT stored on SortPreservingMergeExec or CoalescePartitionsExec. This could make a query return more rows than requested.

What changes are included in this PR?

• Consume a removed fetch when a replacement merge operator is inserted.
• Remember the outermost removed fetch-capable distribution operator and rebuild it around the optimized child when no replacement consumes its limit.
• Preserve the minimum effective fetch across nested distribution operators.
• Carry a fetched ordered merge's limit to a replacement sort when order-preserving variants are removed.

What is the testing strategy for this PR?

Added targeted physical optimizer regression tests covering:

• reoptimizing a fetched SortPreservingMergeExec;
• reoptimizing a fetched CoalescePartitionsExec;
• moving a fetched ordered merge's limit to a replacement sort;
• updating an existing fetched single-partition merge snapshot that previously
  encoded the incorrect removal of its limit.

Validated with:

# Fails on e4cf35cbc (current main before this patch): both regression tests fail
cargo test -p datafusion --test core_integration physical_optimizer::enforce_distribution::preserve_fetch_when_reoptimizing

# Passes with this patch
cargo test -p datafusion --test core_integration physical_optimizer::enforce_distribution::preserve_fetch_when_reoptimizing
cargo test -p datafusion --test core_integration physical_optimizer::enforce_distribution::move_fetch_to_replacement_sort
cargo test -p datafusion --test core_integration physical_optimizer::enforce_distribution::test_replace_order_preserving_variants_with_fetch
cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings
RUST_BACKTRACE=1 cargo test --profile ci --exclude datafusion-examples --exclude datafusion-benchmarks --exclude datafusion-cli --workspace --lib --tests --bins --features avro,json,backtrace,extended_tests,recursive_protection,parquet_encryption

Are there any user-facing changes?

Queries preserve their requested global limit when physical distribution requirements are optimized more than once. There are no changes to SQL behavior other than fixing the incorrect result, and no changes to documented public APIs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.35802% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.52%. Comparing base (e4cf35c) to head (1d957a5).

Files with missing lines	Patch %	Lines
...er/src/ensure_requirements/enforce_distribution.rs	91.35%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24809   +/-   ##
=======================================
  Coverage   81.52%   81.52%           
=======================================
  Files        1123     1123           
  Lines      406148   406191   +43     
  Branches   406148   406191   +43     
=======================================
+ Hits       331124   331166   +42     
+ Misses      55659    55658    -1     
- Partials    19365    19367    +2     

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