Use local variable instead of instance variable for RTVI prepend decision

Replace _rtvi_external instance variable with a local prepend_rtvi flag
since it is only used during __init__ to decide whether to prepend the
RTVIProcessor to the pipeline.

src/pipecat/pipeline/task.py

@@ -330,7 +330,7 @@ class PipelineTask(BasePipelineTask):
 
         # RTVI support
         self._rtvi = None
-        self._rtvi_external = False
+        prepend_rtvi = False
         external_rtvi = self._find_processor(pipeline, RTVIProcessor)
         external_observer_found = any(isinstance(o, RTVIObserver) for o in observers)
 
@@ -350,10 +350,10 @@ class PipelineTask(BasePipelineTask):
                 "They are both added by default, no need to add them yourself."
             )
             self._rtvi = external_rtvi
-            self._rtvi_external = True
         elif enable_rtvi:
             self._rtvi = rtvi_processor or RTVIProcessor()
             observers.append(self._rtvi.create_rtvi_observer(params=rtvi_observer_params))
+            prepend_rtvi = True
 
         if self._rtvi:
             # Automatically call RTVIProcessor.set_bot_ready()
@@ -393,10 +393,7 @@ class PipelineTask(BasePipelineTask):
         # Only prepend the RTVIProcessor if we created it ourselves. When the
         # user already placed it inside their pipeline we must not insert it
         # again or it will appear twice in the frame chain.
-        if self._rtvi and not self._rtvi_external:
-            processors = [self._rtvi, pipeline]
-        else:
-            processors = [pipeline]
+        processors = [self._rtvi, pipeline] if prepend_rtvi else [pipeline]
         self._pipeline = Pipeline(processors, source=source, sink=sink)
 
         # The task observer acts as a proxy to the provided observers. This way,