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/mention.rs

@@ -6,6 +6,7 @@ use std::{
     fmt,
     ops::Range,
     path::{Path, PathBuf},
+    str::FromStr,
 };
 use ui::{App, IconName, SharedString};
 use url::Url;
@@ -224,6 +225,14 @@ impl MentionUri {
     }
 }
 
+impl FromStr for MentionUri {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> anyhow::Result<Self> {
+        Self::parse(s)
+    }
+}
+
 pub struct MentionLink<'a>(&'a MentionUri);
 
 impl fmt::Display for MentionLink<'_> {