Projection field metadata is lost during physical plan optimization #24721

Describe the bug

• Filed from fix: preserve projection metadata during optimization #24670 for @gene-bordegaray

Field metadata on a projection's output schema (for example Arrow extension type metadata, or custom key/value metadata added via a select with an alias) can silently disappear when the physical optimizer removes or rewrites ProjectionExec nodes.

The user-visible effects are:

• The schema of query results loses field-level metadata that the plan declared (e.g. extension types like arrow.uuid are stripped)
• Expressions that read field metadata return wrong results, because they see the underlying source field instead of the projected field that carried the metadata

To Reproduce

Run a query whose plan contains a projection that only adds field metadata, e.g.:

ProjectionExec: i@0 AS i
  output field metadata = {"event_field": "true"}

  DataSourceExec: i
    field metadata = {}

The optimizer removes the projection (it looks like an identity projection), and the output schema no longer has the metadata.

Similarly, with a function that reads field metadata (such as a UDF like arrow_metadata):

ProjectionExec: arrow_metadata(i, 'event_field') AS metadata

  ProjectionExec: i@0 AS i
    output metadata = {"event_field": "true"}

    DataSourceExec: i
      metadata = {}

the projections are collapsed and the function evaluates against the scan field (no metadata) instead of the inner projection field.

Expected behavior

Physical plan optimization should not change the output schema of the plan: field metadata present on projection output fields before optimization should still be present afterwards, and metadata-reading expressions should return the same results with and without optimization (true rather than NULL in the example above).

Additional context

Found while upgrading to DataFusion 55: #24462 (comment)