fix: make ToJson PartialEq<dyn Any> consistent with PartialEq

[0lai0]

Which issue does this PR close?

Closes #4430

Rationale for this change

PartialEq<dyn Any> is the path DataFusion uses to compare Arc<dyn PhysicalExpr> values in the physical planner. The previous impl only compared expr and timezone, so two ToJson expressions that differed only in ignore_null_fields (or historically, timezone) would compare equal through that path, risking incorrect deduplication or caching.

What changes are included in this PR?

Delegate PartialEq<dyn Any> to self == other, which calls the regular PartialEq impl and ensures both paths always compare the same fields.

How are these changes tested?

Three new unit tests in json_funcs::to_json::test:

• test_partial_eq_same identical exprs are equal via both paths
• test_partial_eq_dyn_any_differs_on_timezone different timezone is not equal via dyn Any
• test_partial_eq_dyn_any_differs_on_ignore_null_fields different ignore_null_fields is not equal via dyn Any

[andygrove]

Thanks @0lai0 . Could you fix linter errors?

[andygrove]

LGTM. Thanks @0lai0