fix: type scalar UDF returns as Arrow arrays

[BharatDeva]

Which issue does this PR close?

Closes #1516.

Rationale for this change

The scalar UDF typing currently binds the callable return type to pyarrow.DataType. That makes type checkers expect UDF implementations to return a data type object, even though scalar UDF callables return Arrow arrays containing values of the declared return type.

This shows up with the existing examples/python-udf.py pattern, where a function annotated as returning pa.Array is rejected by mypy.

What changes are included in this PR?

This PR updates the scalar UDF type hints in python/datafusion/user_defined.py so that:

• the UDF callable return type is bounded to pa.Array
• ScalarUDF.__init__ accepts a pa.Field for the resolved return field
• the decorator helper accepts the public pa.DataType | pa.Field return-field input

Runtime behavior is unchanged.

Are these changes tested?

Yes. I ran the following local checks:

• uv tool run ruff@0.15.1 check --config pyproject.toml python/datafusion/user_defined.py examples/python-udf.py
  • Result: All checks passed!
• uv tool run ruff@0.15.1 format --check --config pyproject.toml python/datafusion/user_defined.py examples/python-udf.py
  • Result: 2 files already formatted
• git diff --check
  • Result: passed with no whitespace errors

Are there any user-facing changes?

Yes, for static typing only. User-defined scalar functions that return Arrow arrays should type-check more accurately. There is no runtime API change.

LLM-generated code disclosure

This type-hint update was prepared with assistance from OpenAI Codex and manually reviewed before submission.