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/config.rs

@@ -56,7 +56,7 @@ 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,
@@ -68,7 +68,7 @@ impl Default for TextGenerationConfig {
             model_id: default_text_model_id(),
             prompt: Some(default_prompt().to_owned()),
             temperature: super::super::default_temperature(),
-            max_response_tokens: 16_384,
+            max_response_tokens: Some(16_384),
             max_context_tokens: 128_000,
         }
     }

src/agent/provider/openai/controller.rs

@@ -131,12 +131,18 @@ impl ControllerTrait for Controller {
             .temperature_override
             .unwrap_or(text_generation_config.temperature);
 
-        let request = CreateChatCompletionRequestArgs::default()
-            .max_tokens(text_generation_config.max_response_tokens)
+        let mut request_builder = CreateChatCompletionRequestArgs::default();
+
+        request_builder
             .model(&text_generation_config.model_id)
             .temperature(temperature)
-            .messages(openai_conversation_messages)
-            .build()?;
+            .messages(openai_conversation_messages);
+
+        if let Some(max_response_tokens) = text_generation_config.max_response_tokens {
+            request_builder.max_tokens(max_response_tokens);
+        }
+
+        let request = request_builder.build()?;
 
         if let Ok(request_as_json) = serde_json::to_string(&request) {
             tracing::trace!(