[Bugfix] Fix Qwen3-Omni realtime playback interruption and conversation history (#7791)

Signed-off-by: psv666 <2693925048@qq.com>

docs/serving/realtime_duplex_api.md

@@ -8,12 +8,41 @@ history stays honest, and a dropped connection can resume the same session.
 This page covers how to run a duplex deployment, how to drive it from Python
 with `vllm_omni.clients.duplex.DuplexClient`, and the complete wire contract.
 
-The endpoint is served for models that ship a duplex plugin — currently
-MiniCPM-o 4.5 only — and just for deploy configurations that declare
+The endpoint is served for models that ship a duplex plugin — including
+MiniCPM-o 4.5 and Qwen3-Omni — and just for deploy configurations that declare
 `session_mode: duplex`. PersonaPlex and Nemotron VoiceChat are ported to the
 plugin contract in follow-up PRs and are not served over this endpoint yet. The runtime architecture
 is described in [Full-Duplex Runtime (MiniCPM-o 4.5)](../design/fullduplex.md).
 
+## Qwen3-Omni conversation history
+
+Qwen3-Omni keeps incoming conversation items independently addressable by item
+ID. Before applying the model's chat template, it merges consecutive user
+items into one multimodal user message. An assistant message, including an
+empty message reserved for an interrupted or unheard answer, separates turns.
+The final user message places media before text instructions, following the
+[official Qwen3-Omni demo's `format_history()`](https://github.com/QwenLM/Qwen3-Omni/blob/e4235853125589c789f06a2dd83e9f4126df5e9d/web_demo.py#L84-L179).
+
+A camera image or text item may arrive after an audio commit and before
+`response.create`. Those items belong to the same pending user turn: the
+committed audio remains associated with its history item even when that item
+is no longer last. Prompt preparation snapshots the selected inputs; later
+items do not modify an already prepared request.
+
+The duplex adapter retains its existing budgets: at most four audio payloads
+(including the current input), an 8 MiB encoded-audio history budget (keeping
+at least the newest payload), and at most eight prompt images. The recent
+16-message window is rounded outwards to a complete turn. If any audio in an
+older turn is no longer retained, the entire turn's model context, including
+its images/text and assistant replies, is omitted. Images from that turn are
+therefore **not** permanent session-wide model context. These limits differ
+from the demo's one image-bearing turn and five audio-bearing turns; the
+shared rule is grouping before pruning, rather than copying its UI budgets.
+
+Grouping and pruning affect only the model-input view. They do not merge or
+delete the source conversation items exposed to clients, and do not change
+playback acknowledgement or interruption handling.
+
 ## Quick Start
 
 ### Start the Server