fix: preserve scalar UDF bounds 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 custom evaluate_bounds() behavior across the FFI boundary. The consumer falls back to an unbounded NULL interval, preventing interval analysis from using provider bounds.

What changes are included in this PR?

• Add an evaluate_bounds callback to FFI_ScalarUDF.
• Convert interval inputs and results through the existing FFI_Interval representation.
• Test successful and error results through forced-foreign and dynamic-library round trips.

What is the testing strategy for this PR?

• cargo test -p datafusion-ffi --features integration-tests
• Extended workspace test command from AGENTS.md
• cargo fmt --all -- --check
• cargo clippy -p datafusion-ffi --all-targets --all-features -- -D warnings with only Rust 1.98 compatibility lint allowances

Are there any user-facing changes?

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

Generated-by: Codex (GPT-5)

[emecii]

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

AI-generated with Codex (GPT-5).

[Xuanwo]

Nice fix!