fix(llm): remove max_output_tokens from ChatGPT Subscription payload (#3656)

ChatGPT's Codex API rejects any request that includes max_output_tokens,
returning HTTP 400 "Unsupported parameter: max_output_tokens". This caused
Deep Research to always fail during the endpoint probe when a ChatGPT
Subscription model was selected.

Remove the conditional that set payload["max_output_tokens"] in
_build_chatgpt_responses_payload(). The parameter is simply not sent.

Also update the two affected tests:
- Rename test_chatgpt_subscription_payload_uses_max_output_tokens →
  test_chatgpt_subscription_payload_omits_max_output_tokens
- Rename test_chatgpt_subscription_payload_omits_empty_max_output_tokens →
  test_chatgpt_subscription_payload_omits_max_output_tokens_when_zero
- Assert max_output_tokens is absent rather than present

Fixes #3650

src/llm_core.py

@@ -563,8 +563,9 @@ def _build_chatgpt_responses_payload(
     }
     if not _restricts_temperature(model):
         payload["temperature"] = temperature
-    if max_tokens and max_tokens > 0:
-        payload["max_output_tokens"] = max_tokens
+    # ChatGPT Subscription Codex API does not support max_output_tokens —
+    # passing it returns HTTP 400 "Unsupported parameter: max_output_tokens".
+    # Do not include it in the payload.
     return payload