fix: preserve scalar UDF strictness across FFI

[emecii]

Which issue does this PR close?

• Part of FFI: FFI_ScalarUDF silently drops producer overrides of defaulted trait methods #22330.

Rationale for this change

Foreign scalar UDFs currently lose their is_strict() value across the FFI boundary. The consumer therefore reports false, preventing optimizer rules from using the provider's nullability metadata.

What changes are included in this PR?

• Add an is_strict callback to FFI_ScalarUDF.
• Forward the provider's value through ForeignScalarUDF.
• Test forced-foreign and dynamic-library round trips.

What is the testing strategy for this PR?

• cargo test -p datafusion-ffi --features integration-tests
• cargo fmt --all -- --check

The FFI test suite passes with 122 unit tests plus the integration targets.

Are there any user-facing changes?

Yes. Foreign scalar UDFs now preserve strictness metadata. The FFI_ScalarUDF layout changes, so this PR requires the api change label.

[emecii]

@timsaucer, could you please add the required api change label and trigger CI when convenient?

AI-generated with Codex (GPT-5).

[timsaucer]

This looks very straight forward. Can you clean up the PR description, though? There's a bit of slop in it.

[emecii]

Cleaned up the description and removed the local environment details. Thanks.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.80%. Comparing base (4048898) to head (54f894e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25109      +/-   ##
==========================================
- Coverage   81.80%   81.80%   -0.01%     
==========================================
  Files        1130     1130              
  Lines      417754   417765      +11     
  Branches   417754   417765      +11     
==========================================
  Hits       341754   341754              
- Misses      55875    55881       +6     
- Partials    20125    20130       +5     

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[github-actions]

Thank you for opening this pull request!

Reviewer note: cargo-semver-checks reported the current version number is not SemVer-compatible with the changes in this pull request (compared against the base branch).

Details

     Cloning apache/main
    Building datafusion-ffi v55.0.0 (current)
       Built [  63.252s] (current)
     Parsing datafusion-ffi v55.0.0 (current)
      Parsed [   0.062s] (current)
    Building datafusion-ffi v55.0.0 (baseline)
       Built [  55.548s] (baseline)
     Parsing datafusion-ffi v55.0.0 (baseline)
      Parsed [   0.063s] (baseline)
    Checking datafusion-ffi v55.0.0 -> v55.0.0 (no change; assume patch)
     Checked [   0.244s] 223 checks: 222 pass, 1 fail, 0 warn, 31 skip

--- failure constructible_struct_adds_field: struct exhaustively constructible through public API adds field ---

Description:
A pub struct that could be exhaustively constructed with a literal using only public API has a new pub field, breaking existing exhaustive literals.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field FFI_ScalarUDF.is_strict in /home/runner/work/datafusion/datafusion/datafusion/ffi/src/udf/mod.rs:135

     Summary semver requires new major version: 1 major and 0 minor checks failed
    Finished [ 120.656s] datafusion-ffi