[AURON #2529] Support add_months function natively

[Sigma-Ma]

Which issue does this PR close?

Closes #2529

Rationale for this change

Spark add_months expressions fall back to Spark because AddMonths has no native conversion.

What changes are included in this PR?

• Convert AddMonths to the Spark_AddMonths extension function.
• Reuse Chrono's month arithmetic, with Gregorian cycle reduction to support the full Date32 range.
• Preserve null propagation, month-end adjustment, and version-specific overflow behavior: wrapping in Spark 3.0 and errors in Spark 3.1+.
• Add Rust and Spark regression tests for month arithmetic, scalar and column inputs, date boundaries, overflow, and timestamp-to-date conversion. Spark tests also check native execution.

Are there any user-facing changes?

add_months can now run natively. SQL syntax and result semantics are unchanged.

How was this patch tested?

• cargo test -p datafusion-ext-functions spark_dates::tests::test_spark_add_months --locked.
• ./build/mvn -Ppre -Pspark-3.5 -Pscala-2.12 -pl spark-extension-shims-spark -am install -DskipTests.
• ./build/mvn -Ppre -Pspark-3.5 -Pscala-2.12 -pl spark-extension-shims-spark test -DskipBuildNative -Dsuites=org.apache.auron.AuronFunctionSuite.
• ./dev/reformat --check.

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

• Yes
• No

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