Support images in agent2 threads (#36152)

- Support adding ImageContent to messages through copy/paste and through
path completions
- Ensure images are fully converted to LanguageModelImageContent before
sending them to the model
- Update ACP crate to v0.0.24 to enable passing image paths through the
protocol

Release Notes:

- N/A

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/acp_thread/src/acp_thread.rs

@@ -443,9 +443,8 @@ impl ContentBlock {
                     }),
                 ..
             }) => Self::resource_link_md(&uri),
-            acp::ContentBlock::Image(_)
-            | acp::ContentBlock::Audio(_)
-            | acp::ContentBlock::Resource(_) => String::new(),
+            acp::ContentBlock::Image(image) => Self::image_md(&image),
+            acp::ContentBlock::Audio(_) | acp::ContentBlock::Resource(_) => String::new(),
         }
     }
 
@@ -457,6 +456,10 @@ impl ContentBlock {
         }
     }
 
+    fn image_md(_image: &acp::ImageContent) -> String {
+        "`Image`".into()
+    }
+
     fn to_markdown<'a>(&'a self, cx: &'a App) -> &'a str {
         match self {
             ContentBlock::Empty => "",