[X-3173] Fix union equivalence schema rewrite with stale constants

[xudong963]

Which issue does this PR close?

• Closes #.

Rationale for this change

UnionExec::try_new can panic while computing equivalence properties if stale constant metadata is carried across a projection and then rewritten to the union output schema.

In the observed shape, a filter such as ticker = 'ESU6' can leave a uniform string constant in equivalence properties. After a parent projection drops ticker, union property schema rewriting can see the remaining column slot as a timestamp column and attempt to cast 'ESU6' to Timestamp, which fails during planning.

Equivalence constants are optimizer metadata, so an unrepresentable constant after schema rewrite should be discarded rather than failing query planning.

What changes are included in this PR?

• Drops a uniform constant during EquivalenceProperties::with_new_schema if its value cannot be cast to the rewritten expression type.
• Removes trivial equivalence classes after dropping such constants.
• Propagates UnionExec::compute_properties errors from UnionExec::try_new instead of unwrapping.
• Adds a regression test for union equivalence schema rewrite with an unrepresentable stale constant value.

Are these changes tested?

Yes:

• cargo test -p datafusion-physical-expr test_union_drops_unrepresentable_constant_value_after_schema_rewrite
• cargo test -p datafusion-physical-plan union::tests::test_union
• cargo test -p datafusion-physical-expr equivalence::properties::union::tests
• cargo clippy -p datafusion-physical-expr -p datafusion-physical-plan --all-targets -- -D warnings

Are there any user-facing changes?

No API change. This prevents a planner panic for affected UNION ALL + filter + projection query shapes.