fix: use nested loop join for nested piecewise merge keys

[mikamikasuki]

Which issue does this PR close?

• Closes PiecewiseMergeJoin returns wrong results for List keys with NULL elements and fails on Struct keys #25957.

Rationale for this change

With piecewise merge joins enabled, nested range-key comparisons can return incorrect results when an array contains an inner NULL. Struct range keys can also fail with an unsupported-sort error. Both cases should preserve SQL comparison semantics and execute successfully.

What changes are included in this PR?

• Fall back to NestedLoopJoinExec whenever either piecewise merge join range key has a nested datatype.
• Update expected plans for existing nested-key right semi join cases.
• Add SQL logic coverage for an array comparison containing an inner NULL and for a Struct comparison.

What is the testing strategy for this PR?

• cargo test --test sqllogictests -- piecewise_merge_join_nested_nulls
• cargo test --test sqllogictests -- pwmj
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• Full extended workspace tests from the repository instructions: 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
• git diff --check

Are there any user-facing changes?

Nested range joins return SQL-correct results when nested values contain NULLs, and Struct range keys no longer fail with an unsupported-sort error. There are no public API changes.

[jayzhan211]

Thanks @mikamikasuki

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.66%. Comparing base (c3ef346) to head (b897fd9).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/core/src/physical_planner.rs	78.57%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26043      +/-   ##
==========================================
- Coverage   82.66%   82.66%   -0.01%     
==========================================
  Files        1147     1147              
  Lines      446357   446395      +38     
  Branches   446357   446395      +38     
==========================================
+ Hits       368971   369002      +31     
- Misses      54997    55001       +4     
- Partials    22389    22392       +3     

☔ 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.

[2010YOUY01]

Thanks for the fix.

The root cause seem to be sorting and comparison don't agree in PWMJ, is it possible to fix that directly?

If that's tricky, this PR's approach totally makes sense, but we would keep the issue open, and add a note for the remaining fix to the root cause.