[Frontend] Allow explicit turn-based serving (#7675)

Signed-off-by: Linze-Shi <linzeshi0@gmail.com>

docs/serving/full_duplex_api.md

@@ -22,14 +22,14 @@ capability gates, see the [Realtime Duplex API guide](realtime_duplex_api.md).
 ## Enable Full Duplex
 
 A model is served full duplex when its registered pipeline declares a
-`duplex_plugin` (the model's `DuplexModelPlugin`). That declaration alone
-decides it: `vllm-omni serve` constructs `DuplexOmni` instead of `AsyncOmni`,
-and every surface the server exposes is backed by a duplex session. It serves
+`duplex_plugin` (the model's `DuplexModelPlugin`) and its deploy configuration
+sets `session_mode: duplex`. `vllm serve --omni` constructs `DuplexOmni`
+instead of `AsyncOmni`. It serves
 `/v1/realtime?duplex=1` (and its alias `/v1/duplex`),
 `POST /v1/chat/completions`, `/v1/models` and `/health`; every other
 turn-based HTTP route (speech, batch, embeddings, video, ...) answers "not
-available". Turn-based use of such a model stays available offline through the
-Python API (`Omni` / `AsyncOmni`).
+available". Set `session_mode: turn` to select the ordinary online serving
+stack instead. The model's supported tasks and endpoint restrictions still apply.
 
 `/v1/chat/completions` is the ordinary chat service running on the duplex
 engine: a request is a turn-based generation on the same stages, served
@@ -45,8 +45,24 @@ The deploy configuration of such a model must agree:
 session_mode: duplex
 ```
 
-A duplex model started with a deploy configuration that does not set it fails
-at startup rather than falling back to turn-based serving.
+A duplex-capable model must explicitly set `session_mode` to `duplex` or `turn`
+in its deploy configuration, possibly through `base_config` inheritance.
+Missing or invalid values fail at startup. The default MiniCPM-o deployment
+continues to use duplex mode.
+
+To run MiniCPM-o 4.5 with the turn-based engine:
+
+```bash
+vllm serve openbmb/MiniCPM-o-4_5 --omni \
+  --deploy-config vllm_omni/deploy/minicpmo_4_5_turn.yaml \
+  --trust-remote-code \
+  --port 8091
+```
+
+This profile inherits the default model and stage settings and overrides only
+`session_mode`. It supports ordinary HTTP requests without creating a duplex
+session handler. Mode selection happens at startup; a WebSocket query parameter
+does not switch engines. The Python `Omni` / `AsyncOmni` APIs are unchanged.
 
 !!! warning