fix: block notification permission dialogs via Chrome policy

[rgarcia]

Summary

• Add DefaultNotificationsSetting: 2 Chrome policy to automatically deny all notification permission requests
• Prevents "site wants to show notifications" dialogs from appearing in the automated browser environment

Test plan

• Deploy updated chromium-headful image
• Navigate to a site that requests notification permissions (e.g., yahoo.com)
• Verify no notification permission dialog appears

---

Note

Sets Chrome policy DefaultNotificationsSetting: 2 to automatically deny all notification permission prompts.

• Policies:
  • Update shared/chromium-policies/managed/policy.json:
    • Add DefaultNotificationsSetting: 2 to auto-deny site notification permission requests.

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