fix: handle WindowFunction in ExprFunctionExt filter and distinct on Expr

[duc12111]

Which issue does this PR close?

• Closes partition_by and filter on ExprFunctionExt #21697.

Rationale for this change

In impl ExprFunctionExt for Expr, the filter() and distinct() methods only handle Expr::AggregateFunction. When called as the first method on an Expr::WindowFunction, they create an empty builder (fun: None), losing the expression entirely. .build() then errors with "ExprFunctionExt can only be used with Expr::AggregateFunction or Expr::WindowFunction".

After adding filter/distinct fields to WindowFunctionParams and updated build(), but ot did not update the Expr dispatch. The order_by() and null_treatment() methods already handle both variants correctly.

What changes are included in this PR?

• Updated filter() in impl ExprFunctionExt for Expr to handle both AggregateFunction and WindowFunction variants, following the same pattern used by order_by() and null_treatment().
• Updated distinct() in impl ExprFunctionExt for Expr with the same fix.

Are these changes tested?

Yes

Added 4 unit tests:

• test_window_filter_first — verifies filter() works as first call on a WindowFunction
• test_window_distinct_first — verifies distinct() works as first call on a WindowFunction
• test_window_filter_then_partition_by — verifies chaining filter() then partition_by() on a WindowFunction
• test_aggregate_filter_still_works — regression test confirming aggregates still work

Are there any user-facing changes?

No breaking changes.

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