acp: Better telemetry IDs for ACP agents (#44544)

We were defining these in multiple places and also weren't leveraging
the ids the agents were already providing.

This should make sure we use them consistently and avoid issues in the
future.

Release Notes:

- N/A

crates/agent/src/agent.rs

@@ -947,8 +947,8 @@ impl acp_thread::AgentModelSelector for NativeAgentModelSelector {
 }
 
 impl acp_thread::AgentConnection for NativeAgentConnection {
-    fn telemetry_id(&self) -> &'static str {
-        "zed"
+    fn telemetry_id(&self) -> SharedString {
+        "zed".into()
     }
 
     fn new_thread(

crates/agent/src/native_agent_server.rs

@@ -21,10 +21,6 @@ impl NativeAgentServer {
 }
 
 impl AgentServer for NativeAgentServer {
-    fn telemetry_id(&self) -> &'static str {
-        "zed"
-    }
-
     fn name(&self) -> SharedString {
         "Zed Agent".into()
     }