open_ai: Disable parallel_tool_calls (#28056)

This PR disables `parallel_tool_calls` for the models that support it,
as the Agent currently expects at most one tool use per turn.

It was a bit of trial and error to figure this out. OpenAI's API
annoyingly will return an error if passing `parallel_tool_calls` to a
model that doesn't support it.

Release Notes:

- N/A

crates/language_models/src/provider/cloud.rs

@@ -690,7 +690,7 @@ impl LanguageModel for CloudLanguageModel {
             }
             CloudModel::OpenAi(model) => {
                 let client = self.client.clone();
-                let request = into_open_ai(request, model.id().into(), model.max_output_tokens());
+                let request = into_open_ai(request, model, model.max_output_tokens());
                 let llm_api_token = self.llm_api_token.clone();
                 let future = self.request_limiter.stream(async move {
                     let response = Self::perform_llm_completion(