feat(composite): support Codex endpoints

docs/COMPOSITE_GROUPS.md

@@ -53,6 +53,12 @@ route's `upstream_model` before dispatch. For Gemini native paths such as
 `/v1beta/models/{model}:generateContent`, the gateway resolves `{model}` and
 the handler forwards the resolved upstream model.
 
+Codex Alpha Search and Live requests use the `responses` route domain. Live
+requests resolve the model from `session.model`, including multipart `session`
+payloads, and apply the configured `upstream_model` before dispatch.
+Codex model manifest requests reuse the existing OpenAI account selection and
+failover path within the Composite group.
+
 ## Built-In Detection
 
 Composite routing detects common public model IDs and provider-prefixed IDs: