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 with @2010YOUY01 in #23461 this pr improves the error message when there is a metadata conflict when schemas get merged. It also reverts #23605, so CrossJoinExec has again the original behavior.

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?

• Revert fix: Preserve metadata when a cross-join is swapped #23605 the behavior change for CrossJoinExec
• Improve error messages for schema validation in physical_planner.rs
• Data creation in test_context.rs
• Reproduction in metadata.slt

Are these changes tested?

Yes

Are there any user-facing changes?

Yes, error message improved when the schema metadata has conflicting keys using a cross-join.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.75%. Comparing base (68d5874) to head (a96c221).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/core/src/physical_planner.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #23952    +/-   ##
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  Coverage   80.75%   80.75%            
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  Files        1096     1096            
  Lines      373282   373522   +240     
  Branches   373282   373522   +240     
========================================
+ Hits       301440   301642   +202     
- Misses      53869    53894    +25     
- Partials    17973    17986    +13     

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[mkleen]

@2010YOUY01 Could you do a review please?

[2010YOUY01]

LGTM, thank you!

[mkleen]

LGTM, thank you!

Thanks for the review!