ToJson PartialEq<dyn Any> impl is inconsistent with PartialEq impl

[andygrove]

Describe the bug

In native/spark-expr/src/json_funcs/to_json.rs, ToJson has two equality impls that compare different sets of fields:

impl PartialEq for ToJson {
    fn eq(&self, other: &Self) -> bool {
        self.expr.eq(&other.expr)
            && self.timezone.eq(&other.timezone)
            && self.ignore_null_fields.eq(&other.ignore_null_fields)
    }
}

impl PartialEq<dyn Any> for ToJson {
    fn eq(&self, other: &dyn Any) -> bool {
        if let Some(other) = other.downcast_ref::<ToJson>() {
            self.expr.eq(&other.expr) && self.timezone.eq(&other.timezone)
        } else {
            false
        }
    }
}

The PartialEq<dyn Any> impl omits timezone historically, and after #3875 also omits ignore_null_fields. (Originally only timezone was the discrepancy; the gap widened when ignore_null_fields was added.)

This matters because PartialEq<dyn Any> is what DataFusion uses to compare Arc<dyn PhysicalExpr> values, so two ToJson exprs that differ only in timezone or ignore_null_fields will compare equal through the trait-object path. That can cause incorrect deduplication or caching in the physical planner.

Steps to reproduce

Construct two ToJson exprs with the same child but different timezone or ignore_null_fields, compare them via Arc<dyn PhysicalExpr> equality (e.g. through PhysicalExpr::eq on the Any path), and observe that they compare equal.

Expected behavior

Both equality paths should compare the same fields. The PartialEq<dyn Any> impl should include timezone and ignore_null_fields, matching the regular PartialEq impl. Equivalent fix: derive PartialEq<dyn Any> through the regular PartialEq by downcasting and delegating to self == other.

Additional context

Spotted while reviewing #3875. The PR added ignore_null_fields to the regular PartialEq but did not touch the dyn Any impl, which made an existing inconsistency slightly worse.