Document bounded output repair

docs/integrations/openai-compatible-chat.md

@@ -67,7 +67,8 @@ The client conditionally includes:
 - 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.
+  strict flag, and schema document. Plain JSON validation does not add an
+  object-only response constraint.
 
 The engine resolves backend, profile, and request extra-parameter maps by
 whole-map replacement rather than key merging. The resulting effective map is
@@ -99,10 +100,12 @@ drained.
 
 The bounded body must contain exactly one OpenAI-compatible JSON response
 object followed only by JSON whitespace and EOF. The client returns the first
-choice's non-empty message content and maps prompt, completion, total, cached,
-and cache-write token counts. Invalid or truncated JSON, trailing non-whitespace
-data, a second JSON value, absent choices, empty first-choice content, and size
-overflow are malformed responses and return no partial result.
+choice's explicitly present string message content, including an empty or
+whitespace-only string, and maps prompt, completion, total, cached, and
+cache-write token counts. Invalid or truncated JSON, trailing non-whitespace
+data, a second JSON value, absent choices, missing content, `null` content,
+non-string content, and size overflow are malformed responses and return no
+partial result.
 
 For a non-2xx status, Promptkit recognizes one JSON document with a top-level
 object-valued `error` member. Its optional `message` and `type` fields must be