Support @-mentions in inline assists and when editing old agent panel messages (#29734)

Closes #ISSUE

Co-authored-by: Bennet <bennet@zed.dev>

Release Notes:

- Added support for context `@mentions` in the inline prompt editor and
when editing past messages in the agent panel.

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/agent/src/thread_store.rs

@@ -733,6 +733,8 @@ pub struct SerializedMessage {
     pub tool_results: Vec<SerializedToolResult>,
     #[serde(default)]
     pub context: String,
+    #[serde(default)]
+    pub creases: Vec<SerializedCrease>,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -813,10 +815,19 @@ impl LegacySerializedMessage {
             tool_uses: self.tool_uses,
             tool_results: self.tool_results,
             context: String::new(),
+            creases: Vec::new(),
         }
     }
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+pub struct SerializedCrease {
+    pub start: usize,
+    pub end: usize,
+    pub icon_path: SharedString,
+    pub label: SharedString,
+}
+
 struct GlobalThreadsDatabase(
     Shared<BoxFuture<'static, Result<Arc<ThreadsDatabase>, Arc<anyhow::Error>>>>,
 );