fix(react-ui): stop traces page crash when switching trace tabs (#11387)

Switching from Backend Traces back to API Traces crashed the page
with "can't access property status, e.response is undefined" (#11376).
The API table briefly renders the previous tab's backend rows while the
refetch effect is still pending, and those rows carry no `response`
envelope. The status column dereferenced it unguarded. Render a neutral
placeholder instead of throwing, and cover the tab-switch scenario with
a regression spec.

Assisted-by: opencode:big-pickle

Signed-off-by: Nandana Dileep <110280757+nandanadileep@users.noreply.github.com>

core/http/react-ui/e2e/traces-live.spec.js

@@ -20,3 +20,46 @@ test('marks an API trace with no response status as in progress', async ({ page
   await expect(row.locator('[title="In progress"]')).toBeVisible()
   await expect(row.locator('.fa-check-circle')).toHaveCount(0)
 })
+
+// Regression for #11376: switching from Backend Traces back to API Traces
+// used to crash the page. `traces` holds whichever list was fetched last, so
+// right after `setActiveTab('api')` — before the refetch effect lands — the
+// API table renders the previous tab's backend rows, which carry no
+// `response` envelope. The status column must tolerate that instead of
+// dereferencing `trace.response.status` and tearing down the React tree.
+test('switching from backend to API traces with a response-less row does not crash', async ({ page }) => {
+  const pageErrors = []
+  page.on('pageerror', (e) => pageErrors.push(e.message))
+
+  await page.route('**/api/traces?*', route => route.fulfill({
+    json: [{
+      id: 'api-1',
+      timestamp: '2026-08-05T02:00:00Z',
+      request: { method: 'POST', path: '/v1/chat/completions' },
+      response: { status: 200 },
+    }],
+    headers: { 'X-Total-Count': '1' },
+  }))
+  await page.route('**/api/backend-traces?*', route => route.fulfill({
+    json: [{
+      id: 'backend-1',
+      type: 'llm',
+      timestamp: '2026-08-05T02:00:00Z',
+      model_name: 'mock-model',
+      summary: 'generated a reply',
+    }],
+    headers: { 'X-Total-Count': '1' },
+  }))
+
+  await page.goto('/app/traces')
+  await expect(page.locator('tbody tr').filter({ hasText: '/v1/chat/completions' })).toBeVisible()
+
+  await page.getByRole('button', { name: /Backend Traces/ }).click()
+  await expect(page.locator('tbody tr').filter({ hasText: 'generated a reply' })).toBeVisible()
+
+  await page.getByRole('button', { name: /API Traces/ }).click()
+  // The stale backend row renders in the API table for one frame; the status
+  // column falls back to a neutral placeholder rather than throwing.
+  await expect(page.locator('tbody tr').filter({ hasText: '/v1/chat/completions' })).toBeVisible()
+  expect(pageErrors).toEqual([])
+})

core/http/react-ui/src/pages/Traces.jsx

@@ -667,6 +667,8 @@ export default function Traces() {
                     <td>
                       {trace.response?.status === 0
                         ? <span className="badge badge-info">Running</span>
+                        : trace.response?.status == null
+                        ? <span className="badge badge--soft">-</span>
                         : <span className={`badge ${trace.response.status < 400 ? 'badge-success' : 'badge-error'}`}>{trace.response.status}</span>}
                     </td>
                     <td><LatencyCell ns={trace.duration} max={slowestTrace} /></td>