[SPARK-59042][SQL] Fix PushProjectionThroughUnion failure on correlated scalar subquery over UNION ALL

[zahed1994]

What changes were proposed in this pull request?

This PR fixes a java.util.NoSuchElementException when PushProjectionThroughUnion optimizes a Project over a Union where project expressions contain subqueries or outer references.

Specifically:

• Updated PushProjectionThroughUnion.pushToRight to use rewrites.getOrElse(a, a) instead of rewrites(a) when looking up attribute references. This prevents NoSuchElementException when encountering subquery-internal or unmapped attributes.
• Added updateOuterReferencesInSubquery helper in PushProjectionThroughUnion to recurse into subquery plans (PlanExpression.plan) and rewrite inner OuterReference(a) references to match the right child's corresponding attributes.

Why are the changes needed?

When a query contains a correlated scalar subquery (or any expression referencing subquery-internal attributes) over a UNION ALL, PushProjectionThroughUnion attempts to push project expressions to the children of the Union.

Prior to this fix:

1. pushToRight called rewrites(a) directly on all attributes. Subquery-internal attributes (e.g. r.x#7 in WHERE r.x = u.a) were absent from rewrites (which maps left.output -> right.output), throwing java.util.NoSuchElementException: key not found: x#7.
2. Outer references inside subquery plans (OuterReference(a)) were skipped by e transform because OuterReference extends LeafExpression and PlanExpression.plan is a LogicalPlan rather than an Expression child.

Does this PR introduce any user-facing change?

No. Fixes an optimization failure exception for correlated subqueries over UNION ALL.

How was this patch tested?

• Added catalyst optimizer unit test in PushProjectThroughUnionSuite.
• Added end-to-end SQL query test in SubquerySuite.

Was this patch authored or co-authored using generative AI tooling?

No.