fix: reborrow metadata values when intersecting union metadata

[officialasishkumar]

Which issue does this PR close?

• Closes intersect_metadata_for_union fails to typecheck when rkyv is in scope on rustc 1.91.1 #22488.

Rationale for this change

intersect_metadata_for_union compares values retained by HashMap::retain with values from another metadata map. The retained value is passed as &mut String, which can make Option equality ambiguous for downstream crates when additional blanket PartialEq implementations are in scope.

Reborrowing the retained value as &String keeps the comparison type explicit without changing behavior.

What changes are included in this PR?

The metadata retain predicate now compares metadata.get(k) with Some(&*v) instead of Some(v).

Are these changes tested?

Yes.

• cargo test -p datafusion-expr intersect_metadata_tests
• cargo fmt --all -- --check
• git diff --check

Are there any user-facing changes?

No runtime behavior change. This avoids a downstream compilation failure in the reported dependency configuration.