fix: preserve outer join equivalences for null rows

[Prajwal-k-tech]

Which issue does this PR close?

• Closes Wrong ORDER BY results after an outer join when the nullable side equates a column to coalesce/CASE #26036.

Rationale for this change

After a LEFT, RIGHT, or FULL OUTER JOIN, the nullable side can contain rows whose columns were extended with NULLs. An equivalence such as b = coalesce(a, 0) is true on the input rows but does not remain true for those null-extended rows. Retaining it can lead to incorrect ordering and LIMIT results.

What changes are included in this PR?

• Filter nullable-side equivalence classes against a one-row all-NULL batch for the outer-join cases where that side may be null-extended.
• Ignore volatile expressions during this check and discard classes that are no longer meaningful.
• Add LEFT and FULL OUTER JOIN SQL regression cases, including the reported ORDER BY/LIMIT behavior.

What is the testing strategy for this PR?

• Verified the new SQL cases fail on the unmodified code and pass with this change.
• cargo test -p datafusion-sqllogictest --test sqllogictests -- joins
• cargo test -p datafusion-physical-expr (1,685 passed, 2 ignored; 13 doctests passed)
• RUST_BACKTRACE=1 cargo test -p datafusion (passed)
• RUST_BACKTRACE=1 cargo test -p datafusion-cli (passed)
• cargo clippy --all-targets --all-features -- -D warnings (passed)
• cargo +1.99.0 fmt --all -- --check and git diff --check (passed)

This contribution was prepared with AI assistance.

Are there any user-facing changes?

This fixes incorrect query results for affected outer joins. There are no API changes.

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[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-physical-expr v55.1.0 (current)
       Built [  29.470s] (current)
     Parsing datafusion-physical-expr v55.1.0 (current)
      Parsed [   0.049s] (current)
    Building datafusion-physical-expr v55.1.0 (baseline)
       Built [  28.554s] (baseline)
     Parsing datafusion-physical-expr v55.1.0 (baseline)
      Parsed [   0.051s] (baseline)
    Checking datafusion-physical-expr v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.349s] 223 checks: 222 pass, 1 fail, 0 warn, 31 skip

--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/method_parameter_count_changed.ron

Failed in:
  datafusion_physical_expr::equivalence::EquivalenceGroup::join takes 4 parameters in /home/runner/work/datafusion/datafusion/target/semver-checks/git-apache_main/705d2e66e6058a9682221d3e6bffabc69ba10efe/datafusion/physical-expr/src/equivalence/class.rs:812, but now takes 5 parameters in /home/runner/work/datafusion/datafusion/datafusion/physical-expr/src/equivalence/class.rs:816

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [  59.637s] datafusion-physical-expr
    Building datafusion-sqllogictest v55.1.0 (current)
       Built [  96.372s] (current)
     Parsing datafusion-sqllogictest v55.1.0 (current)
      Parsed [   0.016s] (current)
    Building datafusion-sqllogictest v55.1.0 (baseline)
       Built [  96.396s] (baseline)
     Parsing datafusion-sqllogictest v55.1.0 (baseline)
      Parsed [   0.018s] (baseline)
    Checking datafusion-sqllogictest v55.1.0 -> v55.1.0 (no change; assume patch)
     Checked [   0.112s] 223 checks: 223 pass, 31 skip
     Summary no semver update required
    Finished [ 196.784s] datafusion-sqllogictest

[Prajwal-k-tech]

I pushed a regression fix for the CI failure: zero-column batches now preserve their row count, with a focused Full-join test. GitHub marked the new fork workflow runs as action_required before starting jobs. Could a maintainer approve them so CI can verify the updated commit?