Replace llm_completion_user_turn_stop_strategies() with FilterIncompleteUserTurnStrategies

Wrap the detector chain with `deferred(...)` and append the LLM
completion gate via a `UserTurnStrategies` specialization rather than
a free-standing helper, mirroring the existing
`ExternalUserTurnStrategies` pattern. The class lives next to other
strategy containers in `pipecat.turns.user_turn_strategies`, so users
discover it where they're already configuring `user_turn_strategies`.

The deprecated `filter_incomplete_user_turns` flag now rewires
through `FilterIncompleteUserTurnStrategies` under the hood, keeping
the migration path identical to before. `deferred(...)` stays public
as the explicit escape hatch for non-default compositions.

tests/test_context_aggregators_universal.py

@@ -64,7 +64,10 @@ from pipecat.turns.user_mute import (
     MuteUntilFirstBotCompleteUserMuteStrategy,
 )
 from pipecat.turns.user_stop import SpeechTimeoutUserTurnStopStrategy
-from pipecat.turns.user_turn_strategies import UserTurnStrategies
+from pipecat.turns.user_turn_strategies import (
+    FilterIncompleteUserTurnStrategies,
+    UserTurnStrategies,
+)
 from pipecat.utils.text.base_text_aggregator import AggregationType
 
 USER_TURN_STOP_TIMEOUT = 0.2
@@ -179,15 +182,9 @@ class TestLLMUserAggregator(unittest.IsolatedAsyncioTestCase):
         assert context.messages[0]["content"] == "Hi there!"
 
     async def test_llm_messages_update_does_not_inject_turn_completion_into_context(self):
-        from pipecat.turns.user_turn_strategies import (
-            llm_completion_user_turn_stop_strategies,
-        )
-
         context = LLMContext()
         params = LLMUserAggregatorParams(
-            user_turn_strategies=UserTurnStrategies(
-                stop=llm_completion_user_turn_stop_strategies(),
-            ),
+            user_turn_strategies=FilterIncompleteUserTurnStrategies(),
         )
         pipeline = Pipeline([LLMUserAggregator(context, params=params)])