feat(datafusion): support isnan predicate pushdown to Iceberg

[charlesdong1991]

Which issue does this PR close?

Part of the ongoing effort to improve predicate pushdown coverage in the DataFusion integration.

• Closes Add isnan predicate support from Datafusion #2143

What changes are included in this PR?

This PR adds support for pushing down isnan() predicates from DataFusion to Iceberg's native IsNan / NotNan predicate operators.

In DataFusion, isnan() is represented as a scalar function (Expr::ScalarFunction) rather than a dedicated Expr variant (unlike IsNull / IsNotNull). This PR introduces a new scalar_function_to_iceberg_predicate helper in expr_to_predicate.rs that matches scalar functions by name at runtime and converts isnan(col) into Predicate::Unary(IsNan, col).

Negation (NOT isnan(col)) is handled automatically: the existing Expr::Not arm wraps the result in Predicate::Not(...), and Iceberg's downstream rewrite_not visitor normalizes it into Predicate::Unary(NotNan, col).

This enables file pruning using nan_value_counts in manifest metadata for float/double columns, as defined in the Iceberg spec — Manifest Files: field summaries and column statistics.

Are these changes tested?

Yes