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/assistant_context/Cargo.toml

@@ -11,6 +11,9 @@ workspace = true
 [lib]
 path = "src/assistant_context.rs"
 
+[features]
+test-support = []
+
 [dependencies]
 agent_settings.workspace = true
 anyhow.workspace = true

crates/assistant_context/src/context_store.rs

@@ -138,6 +138,27 @@ impl ContextStore {
         })
     }
 
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn fake(project: Entity<Project>, cx: &mut Context<Self>) -> Self {
+        Self {
+            contexts: Default::default(),
+            contexts_metadata: Default::default(),
+            context_server_slash_command_ids: Default::default(),
+            host_contexts: Default::default(),
+            fs: project.read(cx).fs().clone(),
+            languages: project.read(cx).languages().clone(),
+            slash_commands: Arc::default(),
+            telemetry: project.read(cx).client().telemetry().clone(),
+            _watch_updates: Task::ready(None),
+            client: project.read(cx).client(),
+            project,
+            project_is_shared: false,
+            client_subscription: None,
+            _project_subscriptions: Default::default(),
+            prompt_builder: Arc::new(PromptBuilder::new(None).unwrap()),
+        }
+    }
+
     async fn handle_advertise_contexts(
         this: Entity<Self>,
         envelope: TypedEnvelope<proto::AdvertiseContexts>,