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/openai_compat/mod.rs

@@ -56,7 +56,7 @@ pub fn default_config() -> Config {
 
     if let Some(text_generation) = &mut config.text_generation {
         text_generation.model_id = "some-model".to_string();
-        text_generation.max_response_tokens = 4096;
+        text_generation.max_response_tokens = Some(4096);
         text_generation.max_context_tokens = 128_000;
     }