Fully support all mention kinds (#36134)

Feature parity with the agent1 @mention kinds:
- File
- Symbols
- Selections
- Threads
- Rules
- Fetch


Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent/src/thread_store.rs

@@ -205,6 +205,22 @@ impl ThreadStore {
         (this, ready_rx)
     }
 
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn fake(project: Entity<Project>, cx: &mut App) -> Self {
+        Self {
+            project,
+            tools: cx.new(|_| ToolWorkingSet::default()),
+            prompt_builder: Arc::new(PromptBuilder::new(None).unwrap()),
+            prompt_store: None,
+            context_server_tool_ids: HashMap::default(),
+            threads: Vec::new(),
+            project_context: SharedProjectContext::default(),
+            reload_system_prompt_tx: mpsc::channel(0).0,
+            _reload_system_prompt_task: Task::ready(()),
+            _subscriptions: vec![],
+        }
+    }
+
     fn handle_project_event(
         &mut self,
         _project: Entity<Project>,