[Bug] next_day rejects STRING and TIMESTAMP start dates that Spark accepts #23889

Describe the bug

datafusion-spark's next_day declares
Signature::exact(vec![DataType::Date32, DataType::Utf8], Volatility::Immutable)
in datafusion/spark/src/function/datetime/next_day.rs.

Spark's NextDay extends ImplicitCastInputTypes with
inputTypes = Seq(DateType, StringTypeWithCollation(...)), so Spark casts the
first argument to DATE before evaluating. A STRING or a TIMESTAMP start
date is accepted. DataFusion raises a planning error instead.

This affects Spark 3.5.8 through 4.2.0 identically. The behavior has not
changed across those versions.

To Reproduce

Spark SQL, verified against a live pyspark==4.2.0 under both
spark.sql.ansi.enabled=true and =false:

SELECT next_day('2015-07-23', 'Mon');
-- 2015-07-27

SELECT next_day(TIMESTAMP '2015-07-23 12:12:12', 'Mon');
-- 2015-07-27

SELECT next_day('2015-07-23 12:12:12', 'Mon');
-- 2015-07-27

Spark's own golden file
sql/core/src/test/resources/sql-tests/results/date.sql.out asserts all three.

DataFusion SQL:

SELECT next_day('2015-07-23'::string, 'Mon'::string);
-- Error during planning: Failed to coerce arguments to satisfy a call to
-- 'next_day' function: coercion from Utf8View, Utf8View to the signature
-- Exact(Date32, Utf8) failed. No function matches the given name and argument
-- types 'next_day(Utf8View, Utf8View)'.

SELECT next_day('2015-07-23 12:12:12'::timestamp, 'Mon'::string);
-- Error during planning: Failed to coerce arguments to satisfy a call to
-- 'next_day' function: coercion from Timestamp(ns), Utf8View to the signature
-- Exact(Date32, Utf8) failed.

Expected behavior

next_day should accept the argument types Spark accepts and cast them the way
Spark does.

Additional context

Not fixed in the audit that surfaced this because it is a semantics decision
rather than a mechanical correction. Widening the signature raises questions
the maintainers should answer first:

1. Should datafusion-spark model Spark's ImplicitCastInputTypes generally,
   or does it expect the caller to insert the casts? The answer applies to
   every function in the crate, not just next_day, so a one-off widening here
   would set an undeclared precedent.
2. Spark's STRING-to-DATE cast is itself ANSI dependent. next_day('xx', 'Mon')
   raises CAST_INVALID_INPUT under ANSI and returns NULL otherwise. Any
   widening has to route through a Spark-compatible cast, not
   arrow::compute::cast.
3. datafusion/sqllogictest/test_files/spark/datetime/next_day.slt currently
   asserts the coercion error as expected behavior. That assertion has to be
   replaced, not merely extended.

The blocked cases are checked in as commented-out queries in
datafusion/sqllogictest/test_files/spark/datetime/next_day.slt, carrying the
Spark 4.2.0 results above. Uncommenting them is the contract for verifying a
fix.

Relevant file and line:
datafusion/spark/src/function/datetime/next_day.rs, SparkNextDay::new.

Surfaced by the audit-datafusion-spark-expression skill.