[AURON #2501] Support datediff function natively

[Sigma-Ma]

Which issue does this PR close?

Closes #2501

Rationale for this change

Spark DateDiff expressions are not currently converted to native expressions, so queries using datediff remain on the fallback path.

This change adds native execution while preserving Spark's argument order, signed day difference, and null behavior.

What changes are included in this PR?

• convert Spark DateDiff expressions in NativeConverters
• register the Spark_DateDiff extension function
• implement the signed Date32 day difference with null propagation
• add Rust unit tests for positive, negative, zero, leap-year, year-boundary, and null cases
• add regression coverage in AuronFunctionSuite

Are there any user-facing changes?

No. The SQL syntax and result semantics remain unchanged. Eligible datediff expressions can now execute natively.

How was this patch tested?

cargo test -p datafusion-ext-functions spark_dates::tests::test_spark_datediff -- --exact

cargo test -p datafusion-ext-functions

./dev/reformat --check

./auron-build.sh --pre --clean true --sparkver 3.5 --scalaver 2.12 --skiptests false -DwildcardSuites=org.apache.auron.AuronFunctionSuite

Was this patch authored or co-authored using generative AI tooling?

• Yes
• No

Generated-by: OpenAI Codex (GPT-5)

ASF guidance: https://www.apache.org/legal/generative-tooling.html