Upgrade async-openai from our own etkecc fork to upstream's 0.31.1

Switches `async-openai` from our own etkecc fork (0.28.1-patched) to the
official crates.io version 0.31.1.

We adapt to async-openai's types reorganization and making use of crate
features to only enable what we need.

src/agent/provider/openai/utils.rs

@@ -1,8 +1,12 @@
 use async_openai::types::{
-    ChatCompletionRequestAssistantMessageArgs, ChatCompletionRequestMessage,
-    ChatCompletionRequestMessageContentPartImage, ChatCompletionRequestSystemMessageArgs,
-    ChatCompletionRequestUserMessageArgs, ChatCompletionRequestUserMessageContent,
-    ChatCompletionRequestUserMessageContentPart, ImageUrlArgs,
+    chat::{
+        ChatCompletionRequestAssistantMessageArgs, ChatCompletionRequestMessage,
+        ChatCompletionRequestMessageContentPartImage,
+        ChatCompletionRequestSystemMessageArgs,
+        ChatCompletionRequestUserMessageArgs, ChatCompletionRequestUserMessageContent,
+        ChatCompletionRequestUserMessageContentPart,
+        ImageUrlArgs,
+    },
 };
 
 use crate::conversation::llm::{