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/config.rs

@@ -66,7 +66,7 @@ pub struct TextGenerationConfig {
     pub temperature: f32,
 
     #[serde(default)]
-    pub max_response_tokens: u32,
+    pub max_response_tokens: Option<u32>,
 
     #[serde(default)]
     pub max_context_tokens: u32,
@@ -78,7 +78,7 @@ impl Default for TextGenerationConfig {
             model_id: default_text_model_id(),
             prompt: Some(default_prompt().to_owned()),
             temperature: super::super::default_temperature(),
-            max_response_tokens: 4096,
+            max_response_tokens: Some(4096),
             max_context_tokens: 128_000,
         }
     }