[fix](rbo) Guard union equal-set propagation

[morrySnow]

Problem

UNION ALL could publish output-column equality that does not hold for every row. Downstream rules may then remove a required window ordering key and change RANK() results.

Two forms reproduce the problem:

1. Regular children project columns in a different order from their internal output. With child filters proving a = b, a union that projects (c, a, b) can incorrectly map that equality to (c, a) and remove a from ORDER BY c, a.
2. A regular child proves a = b, but constant rows such as (1, 2) and (2, 1) do not. The union can still claim the output columns are equal and remove b from ORDER BY a, b.

Root cause

Logical and physical unions independently mapped child equality sets through child.getOutput() positions. The authoritative union ordinal mapping is regularChildrenOutputs, which can have a different order. The derivation also considered only regular children and ignored every constant row carried by the union.

Reproduction

Create rows (1, 1, 10), (2, 2, 10), and (1, 1, 20), duplicate a filtered (c, a, b) child with UNION ALL, and compute RANK() OVER (ORDER BY c, a). The two rows with (c, a) = (10, 2) must have rank 3.

Separately, union filtered table rows satisfying a = b with constant rows (1, 2) and (2, 1), then compute RANK() OVER (ORDER BY a, b). The four ordered pairs must receive ranks 1, 2, 3, and 4.

Fix

• Share one equal-set derivation between logical and physical unions.
• Validate and use each regular child's explicit union-output mapping.
• Intersect child equality classes by per-ordinal class signatures, avoiding pairwise quadratic candidate generation.
• Refine candidates against every constant row after SQL comparison coercion and constant folding.
• Accept only a folded TRUE; NULL, unsupported coercion, non-foldable expressions, and malformed mappings conservatively provide no equality proof.
• Build each surviving equivalence class with linear star edges.

Tests

• ./run-fe-ut.sh --run org.apache.doris.nereids.properties.EqualSetTest (13 tests passed)
• DISABLE_BUILD_UI=ON ./build.sh --fe (passed; Checkstyle reported zero violations)
• ./run-regression-test.sh --run -f regression-test/suites/nereids_rules_p0/union_equal_set/union_equal_set.groovy (1 suite passed, 0 failed, 0 fatal)

Unit coverage exercises both logical and planner-produced physical unions, reordered mappings, multiple constant rows, one violating row, constant folding, cross-type numeric equality, and NULL. Regression coverage checks both retained window order keys and exact results.

[morrySnow]

run buildall

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[hello-stephen]

FE UT Coverage Report

Increment line coverage 79.43% (112/141) 🎉
Increment coverage report
Complete coverage report

[morrySnow]

run buildall