fix: Improve error message for metadata conflict in schema

[mkleen]

Which issue does this PR close?

• Closes Join swap in join_selection flips schema-level metadata, failing "Schema mismatch" #23461

Rationale for this change

Based on the discussion in #23461 we decided to just improves the error message, so the user knows there is a metadata conflict in the schema. It also reverts #23605, so CrossJoinExec has it's original behaviour. This changes a metadata conflict from a user perspective from:

select * from larger_table cross join smaller_table;

 DataFusion error: Internal error: PhysicalOptimizer rule 'join_selection' failed. Schema mismatch. Expected original schema: Field { "a": Int32 }, Field { "b": Int32 }, got new schema: Field { "a": Int32 }, Field { "b": Int32 }

to:

select * from larger_table cross join smaller_table;

DataFusion error: PhysicalOptimizer rule 'join_selection' failed. Schema mismatch caused by Internal error: Schema metadata mismatch: Expected original metadata: {"metadata_key": "right"}, got metadata: {"metadata_key": "left"}.

What changes are included in this PR?

• Improve error message for schema validation in physical_planner.rs
• Data creation in test_context.rs
• Reproduction in slt
• Revert behaviour of CrossJoinExec

Are these changes tested?

Yes

Are there any user-facing changes?

Yes, error message improved when the metadata has a conflict.