AsyncFuncExpr drops async UDF return field metadata

[Kontinuation]

Describe the bug

AsyncFuncExpr rebuilds the output Field for async scalar UDFs from only the field name, data type, and nullability. This drops any metadata attached by the UDF's return_field_from_args(...).

This causes async scalar UDF result batches to lose extension metadata that is present during planning.

I found this while implementing an async UDF version of RS_FromPath in apache/sedona-db for loading raster data using GDAL:

• feat(rust/sedona-raster-gdal): add RS_FromPath sedona-db#831

In that case, the async UDF returned a field representing raster data with extension metadata, but the collected result batches lost that metadata, which broke downstream logic that depended on the logical type.

To Reproduce

A minimal repro is an async scalar UDF that:

1. returns a normal Utf8 value
2. overrides return_field_from_args(...) to attach metadata such as:
   • ARROW:extension:name = test.async.extension

Then run:

SELECT async_extension(value) AS result FROM test_table

and inspect the collected batch schema for result.

Without a fix, the field metadata is missing from the result batch schema.

Expected behavior

The async UDF result field in collected batches should preserve the metadata computed by return_field_from_args(...).

Additional context

Root cause appears to be AsyncFuncExpr::field(...) in datafusion/physical-expr/src/async_scalar_function.rs, which reconstructs a new Field instead of preserving the already planned return_field.