Align scalar UDF literal args with coerced return fields

[Kevin-Li-2025]

Which issue does this PR close?

• Closes ReturnFieldArgs.scalar_arguments type doesn't match with arg_fields #19982.

Rationale for this change

ReturnFieldArgs exposes both coerced arg_fields and optional scalar literal arguments to scalar UDFs. For coerced literal calls such as test_udf(1) where the UDF signature coerces the argument to Int16, the field metadata is Int16 but the scalar argument was not aligned with that field. On current main, the coerced expression can also appear as Cast(Literal), causing scalar_arguments to be None even though the argument is still a literal.

What changes are included in this PR?

• Build scalar UDF ReturnFieldArgs.scalar_arguments from literals after aligning them to the verified/coerced argument fields.
• Recognize literals wrapped in Cast / TryCast so coerced constants are still reported as scalar arguments.
• Add a regression test that asserts arg_fields[0].data_type() matches the scalar argument data type for a coercible scalar UDF.

Are these changes tested?

• cargo fmt --all --check
• CARGO_HOME=/Users/yinxiaogou/Documents/github/.cargo-home CARGO_TARGET_DIR=/Users/yinxiaogou/Documents/github/datafusion-target cargo test -p datafusion --test user_defined_integration user_defined::user_defined_scalar_functions::test_return_field_args_scalar_argument_types_match_arg_fields -- --exact

I also ran cargo test -p datafusion --test user_defined_integration user_defined::user_defined_scalar_functions; the new test and 22 other tests passed, while 3 existing tests failed locally because ARROW_TEST_DATA / PARQUET_TEST_DATA submodule data was unavailable in this checkout.