feat: Add support for Spark NaNvl math expression

[rafafrdz]

Summary

• Add support for Spark's NaNvl expression (returns first arg if not NaN, otherwise second arg)
• Delegates to DataFusion's built-in nanvl function via CometScalarFunction
• No native Rust code needed — DataFusion's function registry handles execution
• SQL test added covering NaN, normal values, NULL, and mixed inputs

Test plan

• SQL test added: nanvl.sql
• Run CometSqlFileTestSuite to verify results match Spark

Related to #2084

[rafafrdz]

Closing this PR — DataFusion's nanvl has different NULL handling semantics than Spark's NaNvl. For the row nanvl(4.0, NULL), DataFusion returns NULL while Spark returns 4.0. This function needs a custom Comet implementation instead of simple delegation.