[AURON #2524] Support unix_date and date_from_unix_date natively

[Sigma-Ma]

Which issue does this PR close?

Closes #2524

Rationale for this change

Spark's unix_date and date_from_unix_date expressions currently fall back to Spark evaluation.

What changes are included in this PR?

• Convert both expressions to the existing native Date32/Int32 casts through Spark shims.
• Keep Spark 3.0 builds compatible, since these functions were introduced in Spark 3.1.
• Add Spark regression tests for nulls, round trips and integer boundaries with ANSI mode enabled and disabled, and Rust tests for array and scalar casts.

Are there any user-facing changes?

Both functions can now execute natively. Their SQL syntax and results are unchanged.

How was this patch tested?

• cargo test -p datafusion-ext-exprs cast::test::test_epoch_date_cast -- --exact.
• ./build/mvn -Ppre -Pspark-3.5 -Pscala-2.12 -pl spark-extension-shims-spark test -Dsuites=org.apache.auron.AuronFunctionSuite -DskipTests=false -DskipBuildNative.
• ./dev/reformat --check.

The new Spark tests failed on the original code because the projection fell back to Spark, and passed after the change.

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

• Yes
• No

Generated-by: OpenAI Codex (GPT-6)

[slfan1989]

LGTM