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/entity/image.rs

@@ -56,12 +56,8 @@ impl ImageSource {
     }
 }
 
-impl From<ImageSource> for async_openai::types::ImageInput {
+impl From<ImageSource> for async_openai::types::images::ImageInput {
     fn from(value: ImageSource) -> Self {
-        async_openai::types::ImageInput::from_vec_u8(
-            value.filename,
-            value.bytes,
-            value.mime_type.to_string(),
-        )
+        async_openai::types::images::ImageInput::from_vec_u8(value.filename, value.bytes)
     }
 }