Join swap in `join_selection` flips schema-level metadata, failing "Schema mismatch"

[fangchenli]

Describe the bug

When join reordering swaps a join's inputs, the swapped plan keeps a different value for a conflicting schema-level metadata key than the original, so OptimizationInvariantChecker rejects it:

Internal error: PhysicalOptimizer rule 'join_selection' failed. Schema mismatch.

swap_inputs rebuilds the join with inputs reversed and wraps it in a reverting ProjectionExec (reorder_output_after_swap), which restores column order and field metadata but not the schema-level metadata. So reversing two inputs that disagree on a key flips the winner, and the pre/post-swap schemas differ. This is on main, independent of merge direction, and shared by cross / hash / NLJ joins.

To Reproduce

// l: 1000 rows, schema metadata {"pandas": "LEFT"}
// r: 2 rows,    schema metadata {"pandas": "RIGHT"}
ctx.sql("SELECT * FROM l CROSS JOIN r").await?.collect().await?;
// -> Internal error: ... 'join_selection' failed. Schema mismatch.

set datafusion.optimizer.join_reordering = false; makes it pass.

Expected behavior

A swap must preserve the output schema, including schema-level metadata.

Additional context

Found while fixing the cross-join initial-plan case in #23434 / PR #23442, which leaves the shared swap path untouched.