fix: clamp 'minimal' reasoning effort to 'low' on Responses API (#9429)

GPT-5.4 supports none/low/medium/high/xhigh but not 'minimal'.
Users may configure 'minimal' via OpenRouter conventions, which would
cause a 400 on native OpenAI. Clamp to 'low' in the codex_responses
path before sending.

run_agent.py

@@ -6143,6 +6143,12 @@ class AIAgent:
                 elif self.reasoning_config.get("effort"):
                     reasoning_effort = self.reasoning_config["effort"]
 
+            # Clamp effort levels not supported by the Responses API model.
+            # GPT-5.4 supports none/low/medium/high/xhigh but not "minimal".
+            # "minimal" is valid on OpenRouter and GPT-5 but fails on 5.2/5.4.
+            _effort_clamp = {"minimal": "low"}
+            reasoning_effort = _effort_clamp.get(reasoning_effort, reasoning_effort)
+
             kwargs = {
                 "model": self.model,
                 "instructions": instructions,