feat: support Spark-compatible stack function

[comphead]

Which issue does this PR close?

• Part of [EPIC] Complete datafusion-spark Spark Compatible Functions #15914
• Related Comet issue: Support inline and stack generators in GenerateExec datafusion-comet#5125

Rationale for this change

Spark's stack(n, expr1, ..., exprk) separates the values into n rows. It is the standard way to unpivot in Spark SQL, and Comet falls back to Spark for it.

What changes are included in this PR?

• stack in datafusion-spark. A ScalarUDF cannot emit rows, so like json_tuple it returns the n rows as a List<Struct<col0, col1, ...>>, which is the data type of Spark's Generator. As in Spark, the list is not nullable while its elements and fields are. The caller (Comet, or unnest in SQL) expands it.
• The arguments are validated as in Spark's Stack.checkInputDataTypes, with the same messages. n is coerced to INT. The values of a column must have the same Spark type, and NULLs, Arrow types of one Spark type (such as Utf8 and Utf8View) and integer literals that fit are cast to it.
• Primitive columns are copied with a strided loop, and string columns of only scalars (such as unpivot keys) are repeated with concat doubling. Other types use take over the concatenated values. On the new stack bench this is about 3x to 5x faster than interleave.

What is the testing strategy for this PR?

spark/generator/stack.slt ports the cases of Spark's GeneratorFunctionSuite, GeneratorExpressionSuite and TypeCoercionSuite, and of the generators, table-valued-functions and join-lateral golden files. It also covers each kernel path. Unit tests cover an empty batch and the list size overflow, which SQL cannot reach.

Are there any user-facing changes?

A new Spark-compatible stack function. Unlike Spark, n must be an integer literal, so a foldable expression such as 3 - 1 is rejected.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.58865% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.80%. Comparing base (390d210) to head (3991012).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/spark/src/function/generator/stack.rs	87.54%	22 Missing and 13 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #26266    +/-   ##
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  Coverage   82.80%   82.80%            
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  Files        1149     1150     +1     
  Lines      454480   454761   +281     
  Branches   454480   454761   +281     
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+ Hits       376347   376580   +233     
- Misses      55267    55299    +32     
- Partials    22866    22882    +16     

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