Add support for OpenAI's o1 models by making max_response_tokens optional

The other prerequisite seems to be not using a `prompt` (`prompt: null`),
but we already supported this.

It'd be nice to add an optional `max_completion_tokens` parameter as
well, for the benefit of the o1 models, but this is not yet supported by
async-openai.
Possibly tracked here: https://github.com/64bit/async-openai/issues/272

src/agent/provider/openrouter/mod.rs

@@ -14,7 +14,7 @@ pub fn default_config() -> Config {
     if let Some(ref mut config) = config.text_generation.as_mut() {
         config.model_id = "mattshumer/reflection-70b:free".to_owned();
         config.max_context_tokens = 8192;
-        config.max_response_tokens = 2048;
+        config.max_response_tokens = Some(2048);
     }
 
     config