feat: add array_first higher-order array function

[EdsonPetry]

Which issue does this PR close?

• N/A. No dedicated tracking issue; this adds a single self-contained higher-order array function. Happy to file one if preferred.

Rationale for this change

DataFusion already provides higher-order array functions such as array_any_match, array_filter, and array_transform, but there is no direct way to retrieve the first element of an array that satisfies a predicate. Today this requires array_filter followed by array_element(..., 1), which materializes an intermediate filtered array. array_first expresses this directly and rounds out the set of lambda-based array functions.

What changes are included in this PR?

• New higher-order function array_first(array, predicate) (alias list_first) in datafusion-functions-nested, returning the first element for which the lambda predicate returns true:
  • returns null when the array is empty or no element matches;
  • a predicate that evaluates to null for an element is treated as not matching;
  • a matched element that is itself null is returned as null.
• Implemented as a HigherOrderUDFImpl following the existing array-lambda functions, including the standard fast paths (fully-null input) and correct handling of sliced lists, null sublists, and captured outer columns.
• Registration in functions-nested (expr_fn re-export and the default higher-order function list).
• Unit tests, sqllogictest coverage, and regenerated SQL function documentation.

Are these changes tested?

Yes:

• Unit tests in array_first.rs covering match/no-match, empty and null arrays, null-predicate handling, matched-null elements, sliced lists, captured outer columns, and non-primitive element types.
• sqllogictest cases in test_files/array/array_first.slt, including LargeList and the list_first alias.

Are there any user-facing changes?

Yes. A new array function array_first (alias list_first) is available in SQL, with generated documentation under the Array Functions section. There are no breaking changes to existing public APIs.