[SPARK-52354] Add type coercion to UnionLoop

[Pajaraja]

What changes were proposed in this pull request?

Add a new rule to WidenSetOperationTypes to work for UnionLoop.

Why are the changes needed?

To have UNION ALL in recursive CTEs behave in the same way regarding type coercion as UNION ALL outside the context of rCTEs.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New Golden file tests.

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

No.