Simplify wait_complete function (apache#19937)

## Which issue does this PR close?

## Rationale for this change

The current v52 signature `pub async fn wait_complete(self: &Arc<Self>)`
(introduced in apache#19546) is a bit unergonomic. The method requires
`&Arc<DynamicFilterPhysicalExpr>`, but when working with `Arc<dyn
PhysicalExpr>`, downcasting only gives you `&DynamicFilterPhysicalExpr`.
Since you can't convert `&DynamicFilterPhysicalExpr` to
`Arc<DynamicFilterPhysicalExpr>`, the method becomes impossible to call.

The `&Arc<Self>` param was used to check` is_used()` via Arc strong
count, but this was overly defensive.

## What changes are included in this PR?

- Changed `DynamicFilterPhysicalExpr::wait_complete` signature from `pub
async fn wait_complete(self: &Arc<Self>)` to `pub async fn
wait_complete(&self)`.

- Removed the `is_used()` check from `wait_complete()` - this method,
like `wait_update()`, should only be called on filters that have
consumers. If the caller doesn't know whether the filter has consumers,
they should call `is_used()` first to avoid waiting indefinitely. This
approach avoids complex signatures and dependencies between the APIs
methods.

## Are these changes tested?

Yes, existing tests cover this functionality, I removed the "mock"
consumer from `test_hash_join_marks_filter_complete_empty_build_side`
and `test_hash_join_marks_filter_complete` since the fix in
apache#19734 makes is_used check the
outer struct `strong_count` as well.

## Are there any user-facing changes?

The signature of `wait_complete` changed.

(cherry picked from commit bef1368)

Author: LiaCastaneda

datafusion/physical-expr/src/expressions/dynamic_filters.rs

@@ -272,6 +272,10 @@ impl DynamicFilterPhysicalExpr {
     ///
     /// This method will return when [`Self::update`] is called and the generation increases.
     /// It does not guarantee that the filter is complete.
+    ///
+    /// Producers (e.g.) HashJoinExec may never update the expression or mark it as completed if there are no consumers.
+    /// If you call this method on a dynamic filter created by such a producer and there are no consumers registered this method would wait indefinitely.
+    /// This should not happen under normal operation and would indicate a programming error either in your producer or in DataFusion if the producer is a built in node.
     pub async fn wait_update(&self) {
         let mut rx = self.state_watch.subscribe();
         // Get the current generation
@@ -283,17 +287,16 @@ impl DynamicFilterPhysicalExpr {
 
     /// Wait asynchronously until this dynamic filter is marked as complete.
     ///
-    /// This method returns immediately if the filter is already complete or if the filter
-    /// is not being used by any consumers.
+    /// This method returns immediately if the filter is already complete.
     /// Otherwise, it waits until [`Self::mark_complete`] is called.
     ///
     /// Unlike [`Self::wait_update`], this method guarantees that when it returns,
     /// the filter is fully complete with no more updates expected.
-    pub async fn wait_complete(self: &Arc<Self>) {
-        if !self.is_used() {
-            return;
-        }
-
+    ///
+    /// Producers (e.g.) HashJoinExec may never update the expression or mark it as completed if there are no consumers.
+    /// If you call this method on a dynamic filter created by such a producer and there are no consumers registered this method would wait indefinitely.
+    /// This should not happen under normal operation and would indicate a programming error either in your producer or in DataFusion if the producer is a built in node.
+    pub async fn wait_complete(&self) {
         if self.inner.read().is_complete {
             return;
         }

datafusion/physical-plan/src/joins/hash_join/exec.rs

@@ -4633,11 +4633,6 @@ mod tests {
         let dynamic_filter = HashJoinExec::create_dynamic_filter(&on);
         let dynamic_filter_clone = Arc::clone(&dynamic_filter);
 
-        // Simulate a consumer by creating a transformed copy (what happens during filter pushdown)
-        let _consumer = Arc::clone(&dynamic_filter)
-            .with_new_children(vec![])
-            .unwrap();
-
         // Create HashJoinExec with the dynamic filter
         let mut join = HashJoinExec::try_new(
             left,
@@ -4686,11 +4681,6 @@ mod tests {
         let dynamic_filter = HashJoinExec::create_dynamic_filter(&on);
         let dynamic_filter_clone = Arc::clone(&dynamic_filter);
 
-        // Simulate a consumer by creating a transformed copy (what happens during filter pushdown)
-        let _consumer = Arc::clone(&dynamic_filter)
-            .with_new_children(vec![])
-            .unwrap();
-
         // Create HashJoinExec with the dynamic filter
         let mut join = HashJoinExec::try_new(
             left,