Give the synthetic Diagnostics operator its declared read capability

The complete browser failure reached Settings but correctly excluded Diagnostics because its offline session omitted diagnosticsRead. Supply that fixture capability without changing production access checks or authentication.

Contract-Neutral: Offline browser fixture only; no production authorization change.
Change-source: pulse-maintainer

frontend-modern/browser-tests/diagnostics-integration.cjs

@@ -28,6 +28,7 @@ const bootstrap = {
     settingsCapabilities: {
       infrastructureRead: true,
       systemSettingsRead: true,
+      diagnosticsRead: true,
       apiAccessRead: true,
       apiAccessWrite: true,
       authenticationRead: true,