agent_ui: Render error context when failing to spawn agent thread (#41908)

Release Notes:

- Fixed not telling the user what went wrong when spawning ACP agents

crates/agent_ui/src/acp/thread_view.rs

@@ -104,7 +104,7 @@ impl ThreadError {
         {
             Self::AuthenticationRequired(acp_error.message.clone().into())
         } else {
-            let string = error.to_string();
+            let string = format!("{:#}", error);
             // TODO: we should have Gemini return better errors here.
             if agent.clone().downcast::<agent_servers::Gemini>().is_some()
                 && string.contains("Could not load the default credentials")
@@ -113,7 +113,7 @@ impl ThreadError {
             {
                 Self::AuthenticationRequired(string.into())
             } else {
-                Self::Other(error.to_string().into())
+                Self::Other(string.into())
             }
         }
     }
@@ -795,7 +795,8 @@ impl AcpThreadView {
         if let Some(load_err) = err.downcast_ref::<LoadError>() {
             self.thread_state = ThreadState::LoadError(load_err.clone());
         } else {
-            self.thread_state = ThreadState::LoadError(LoadError::Other(err.to_string().into()))
+            self.thread_state =
+                ThreadState::LoadError(LoadError::Other(format!("{:#}", err).into()))
         }
         if self.message_editor.focus_handle(cx).is_focused(window) {
             self.focus_handle.focus(window)