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-desktop-tabs-row.tsx

@@ -54,6 +54,7 @@ import {
   WorkspaceTabIcon,
   type WorkspaceTabPresentation,
 } from "@/screens/workspace/workspace-tab-presentation";
+import { buildDeterministicWorkspaceTabId } from "@/workspace-tabs/identity";
 import {
   buildWorkspaceDesktopTabActions,
   type WorkspaceDesktopTabActions,
@@ -379,7 +380,7 @@ function TabChip({
             <ContextMenuTrigger
               {...(dragHandleProps?.attributes as object | undefined)}
               {...(dragHandleProps?.listeners as object | undefined)}
-              testID={`workspace-tab-${tab.key}`}
+              testID={`workspace-tab-${buildDeterministicWorkspaceTabId(tab.target)}`}
               triggerRef={dragHandleProps?.setActivatorNodeRef as unknown as undefined}
               enabledOnMobile={false}
               style={tabChipStyle}