CrossJoinExec merges schema metadata inconsistently with the logical plan

### Describe the bug

When two inputs carry the same schema-level metadata key with different values, a pure cross join (CrossJoinExec) over them feeding an aggregate fails with:

Internal error: Physical input schema should be the same as the one converted
from logical input schema. Differences:
- schema metadata differs: (physical) {"pandas": "RIGHT-SCHEMA"} vs (logical) {"pandas": "LEFT-SCHEMA"}

Hash/equijoins and filtered nested-loop joins on the same tables work; only the pure cross-product path is affected.

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

xref: #16221 #21127