Derive tab testIDs from the tab target

The tab testID was reading from tabId, which intentionally stays as the
draft id after a draft -> agent handoff so the panel React key remains
stable and the component does not remount. The data-testid was riding
along on that identity and stayed workspace-tab-draft_xxx forever, even
though the tab semantically became an agent. Tests that look for
workspace-tab-agent_* timed out.

Read testIDs from tab.target via buildDeterministicWorkspaceTabId so the
testID reflects what the tab represents. React key is unchanged.

packages/app/src/screens/workspace/workspace-screen.tsx

@@ -82,7 +82,11 @@ import type { WorkspaceTab, WorkspaceTabTarget } from "@/stores/workspace-tabs-s
 import { useKeyboardActionHandler } from "@/hooks/use-keyboard-action-handler";
 import type { KeyboardActionDefinition } from "@/keyboard/keyboard-action-dispatcher";
 import { useCreateFlowStore } from "@/stores/create-flow-store";
-import { normalizeWorkspaceTabTarget, workspaceTabTargetsEqual } from "@/workspace-tabs/identity";
+import {
+  buildDeterministicWorkspaceTabId,
+  normalizeWorkspaceTabTarget,
+  workspaceTabTargetsEqual,
+} from "@/workspace-tabs/identity";
 import {
   getHostRuntimeStore,
   useHostRuntimeClient,
@@ -516,7 +520,7 @@ function MobileWorkspaceTabOption({
   onCloseTabsBelow: (tabId: string) => Promise<void> | void;
   onCloseOtherTabs: (tabId: string) => Promise<void> | void;
 }) {
-  const menuTestIDBase = `workspace-tab-menu-${tab.key}`;
+  const menuTestIDBase = `workspace-tab-menu-${buildDeterministicWorkspaceTabId(tab.target)}`;
   const menuEntries = buildWorkspaceTabMenuEntries({
     surface: "mobile",
     tab,