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.
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