Document per-run session and reasoning overrides

docs/integrations/openai-compatible-chat.md

@@ -47,14 +47,16 @@ Each ordinary message contains its `role` and string `content`. A
 cache-controlled message instead uses a text content block containing `type`,
 `text`, and `cache_control`; an empty cache-control TTL is omitted.
 
-A non-empty session ID is trimmed, checked against the internal domain limit,
-and sent as top-level `session_id`. It is not sent as a session header.
+The effective direct or prompt-rendered session ID is trimmed, limited to 256
+Unicode code points, and sent when nonempty as top-level `session_id`. It is
+never also sent as a session header.
 
 The client conditionally includes:
 
 - `temperature`, `max_tokens`, and `top_p` when non-zero or explicitly
   present;
-- non-empty `service_tier` and `reasoning_effort`; and
+- 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,
   strict flag, and schema document.