feat: support factorial, pmod, and rint expressions

[n0r0shi]

Summary

• Wire three math functions from the datafusion-spark crate (SparkFactorial, SparkPmod, SparkRint) to Comet
• Register in jni_api.rs and add serde mappings via CometScalarFunction in QueryPlanSerde.scala

Test plan

• New tests in CometMathExpressionSuite (factorial, pmod, rint) — all pass
• Spotless formatting check passes
• Rust native build passes

[andygrove]

Serde wiring and UDF registrations look correct for all three expressions.

For the tests, the SQL file framework in spark/src/test/resources/sql-tests/expressions/math/ would be a natural home for these. There are already many math expression SQL files there.

On pmod, it might be worth testing the divide-by-zero case. Spark returns null for pmod(10, 0) with integer types and NaN for floating-point types. Floating-point inputs would also be good to cover since SparkPmod supports them.

On factorial, I like the coverage of negative and boundary values. One small thing: factorial(21) is the first value that returns null due to overflow protection. It could be worth adding that as a test case.