feat: add Spark-compatible atan2 function

[KarpagamKarthikeyan]

Which issue does this PR close?

• Closes Implement Spark-compatible atan2 function #23961
• Part of [EPIC] Complete datafusion-spark Spark Compatible Functions #15914

Rationale for this change

Spark provides atan2(exprY, exprX), which returns the angle in radians between the positive x-axis and the point given by the coordinates (exprX, exprY). It was not yet implemented in datafusion-spark — only an auto-generated test stub existed at spark/math/atan2.slt with its query commented out.

What changes are included in this PR?

• Add SparkAtan2 (implementing ScalarUDFImpl) in datafusion/spark/src/function/math/atan2.rs, computing y.atan2(x) element-wise via the Arrow binary kernel.
• Register it in datafusion/spark/src/function/math/mod.rs.
• Enable the atan2.slt sqllogictest.

The signature is exact(Float64, Float64) -> Float64, following the datafusion-spark convention of only accepting Spark-supported types. NULL in either argument propagates to NULL.

Note: DataFusion core already has an atan2, but this is implemented self-contained rather than delegating — matching the crate convention, where 14 of the 15 existing math functions reimplement rather than wrap core (including abs, ceil, floor, round, rint, all of which also exist in core; only pow delegates, as a special case). This keeps the Spark function library complete and consistent.

Are these changes tested?

Yes — datafusion/sqllogictest/test_files/spark/math/atan2.slt covers:

• standard angles and all four quadrants (atan2 is quadrant-aware via the signs of both arguments),
• both axes and the negative x-axis (atan2's range extends to π, unlike atan),
• NULL propagation (either argument) and NULL-beats-Infinity,
• NaN in every position, including atan2(NaN, Infinity) = NaN (for atan2, NaN propagates even over Infinity — unlike hypot, where Infinity dominates),
• all four infinity quadrants and the one-infinite-argument cases,
• signed zeros (atan2(-0, -1) = -π),
• the array path (including a NULL row).

Are there any user-facing changes?

Yes — adds the Spark-compatible atan2 scalar function to datafusion-spark. No breaking changes to public APIs.

[KarpagamKarthikeyan]

cc @Jefffrey @comphead — a follow-up to my recently merged hypot PR, this adds a Spark-compatible atan2 function to datafusion-spark.

It follows the existing math conventions — exact(Float64, Float64) -> Float64, invoke via make_scalar_function, reimplemented self-contained (matching the 14/15 math functions that reimplement rather than delegate to core). Backed by Rust's f64::atan2.

Covered by sqllogictest cases: all four quadrants, both axes, the negative x-axis (range to π), NULL propagation, NaN in every position (incl. atan2(NaN, Infinity) = NaN — note NaN propagates even over Infinity here, unlike hypot), all four infinity quadrants, signed zeros, and the array path. Closes #23961.

Would appreciate a review when you have a moment. Thanks!

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.70%. Comparing base (74eebbb) to head (5419c96).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/math/atan2.rs	83.87%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23962      +/-   ##
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- Coverage   80.70%   80.70%   -0.01%     
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  Files        1095     1096       +1     
  Lines      372554   372586      +32     
  Branches   372554   372586      +32     
==========================================
+ Hits       300661   300683      +22     
- Misses      53924    53929       +5     
- Partials    17969    17974       +5     

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[Jefffrey]

how is this different from the datafusion functions ver?