[Bug] pmod rejects string arguments that Spark implicitly casts to a numeric type

[andygrove]

Describe the bug

Spark's Pmod declares inputType = NumericType and relies on the analyzer's
string promotion rule, so a string argument is implicitly cast. DataFusion's
SparkPmod uses Signature::numeric(2, Volatility::Immutable), which rejects
strings at planning time.

To Reproduce

Spark 4.2.0 (verified with pyspark==4.2.0), and covered by Spark's own golden
file sql/core/src/test/resources/sql-tests/results/typeCoercion/native/promoteStrings.sql.out:

SELECT pmod('1', CAST(1 AS TINYINT));  -- 0, result type bigint
SELECT pmod('10', CAST(3 AS INT));     -- ansi on: 1 (int); ansi off: 1.0 (double)
SELECT pmod('1', '1');                 -- DATATYPE_MISMATCH.BINARY_OP_WRONG_TYPE

Note that Spark's promotion target differs by mode: with
spark.sql.ansi.enabled = true the string is cast to the other argument's type,
and with it disabled the pair is promoted to double. Two strings are rejected
in both modes.

DataFusion:

SELECT pmod('10'::string, 3::int);
-- Error during planning: For function 'pmod' Utf8 and Int32 are not coercible
-- to a common numeric type.

Expected behavior

A string argument is accepted and coerced the way Spark coerces it.

Scope

This is not specific to pmod. Every datafusion-spark function that uses
Signature::numeric inherits it, and reproducing Spark's mode-dependent
promotion target needs a decision about how far datafusion-spark should go in
emulating Spark's analyzer. Filing it so the gap is visible rather than
proposing a pmod-local workaround.

A commented-out query recording the Spark 4.2.0 result is in
datafusion/sqllogictest/test_files/spark/math/pmod.slt.

Relevant code

datafusion/spark/src/function/math/modulus.rs, SparkPmod::new.

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