Move profile composition to the engine facade

docs/integrations/openai-compatible-chat.md

@@ -53,9 +53,9 @@ never also sent as a session header.
 
 The client conditionally includes:
 
-- `temperature`, `max_tokens`, and `top_p` only when selected by a backend,
-  profile, or runtime override, including an explicit runtime zero; they are
-  absent when unspecified;
+- `temperature`, `max_tokens`, and `top_p` only when selected by a profile or
+  runtime override, including an explicit runtime zero; they are absent when
+  unspecified;
 - non-empty `service_tier` and effective `reasoning_effort`; an explicitly
   disabled reasoning setting is empty and therefore omitted; and
 - `response_format` for JSON Schema structured output, including its name,