Update sample configs for OpenAI (gpt-5) to specify max_completion_tokens, not max_response_tokens

Fixup for b43f61f5ff

src/agent/provider/openai/config.rs

@@ -72,9 +72,9 @@ impl Default for TextGenerationConfig {
             model_id: default_text_model_id(),
             prompt: Some(default_prompt().to_owned()),
             temperature: super::super::default_temperature(),
-            max_response_tokens: Some(16_384),
-            max_completion_tokens: None,
-            max_context_tokens: 128_000,
+            max_response_tokens: None,
+            max_completion_tokens: Some(128_000),
+            max_context_tokens: 400_000,
         }
     }
 }