fix(app): seed changes tab in explorer pane even when setup tab occupies it

The explorer companion pane retention commit (de9e85862) auto-opens the
setup progress tab into the hidden explorer pane. toggleWorkspaceExplorerPane
only seeded the working_diff (Changes) tab when the pane was completely
empty, so once the setup tab landed there, clicking the explorer toggle
could never surface Changes/Files again via that pane's default tab.

Also fixes two e2e testid locators (workspace-new-tab-menu-trigger) and
one unit test assertion (browser automation handler) that assumed a bare
pane root, which predates that same explorer-pane-retention change: the
workspace root is now always a group with a hidden companion pane, so
unscoped locators/assertions must target the visible/named pane.

packages/app/e2e/browser/terminal-stuck-size.spec.ts

@@ -56,7 +56,10 @@ async function readRenderedTerminalSize(page: Page): Promise<RenderedTerminalSiz
 }
 
 async function createTerminalViaMenu(page: Page): Promise<void> {
-  await page.getByTestId("workspace-new-tab-menu-trigger").click();
+  // Workspaces always render a hidden explorer companion pane alongside the
+  // main pane, so an unscoped testid locator matches both; scope to the
+  // visible one.
+  await page.getByTestId("workspace-new-tab-menu-trigger").filter({ visible: true }).click();
   await page.getByTestId("workspace-new-tab-menu-terminal").click();
 }
 
@@ -124,7 +127,9 @@ test.describe("terminal PTY size claim under lost window focus", () => {
     await page.setViewportSize({ width: 1280, height: 900 });
 
     await page.goto(buildHostWorkspaceRoute(getServerId(), harness.workspaceId));
-    await expect(page.getByTestId("workspace-new-tab-menu-trigger")).toBeVisible({
+    await expect(
+      page.getByTestId("workspace-new-tab-menu-trigger").filter({ visible: true }),
+    ).toBeVisible({
       timeout: 30_000,
     });