feat: expose notification retry backoff configuration in admin diagnostics

[diksha78dev]

Description

This PR provides operators with a structured way to inspect the active notification retry policy and delivery backoff settings by exposing a new admin diagnostics endpoint.

Changes included:

• Added NotificationDiagnosticsResponse schema to represent the configuration contract.
• Implemented get_delivery_configuration() helper in notification_service.py.
• Exposed GET /api/v1/admin/diagnostics/notifications route protected by existing admin verification and rate limiting.
• Added integration tests to ensure the endpoint contract is maintained and authentication works as expected.

Related Issues

Fixes #801

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

I wrote a new integration test (test_admin_diagnostics_notifications) and verified the behavior by running pytest testing/backend/integration/test_notification_routes.py.

The test verifies:

• Unauthenticated access returns HTTP 401.
• Authenticated access using a mocked admin_api_key returns HTTP 200.
• The returned JSON payload strictly adheres to the NotificationDiagnosticsResponse contract, containing the correct keys and data types.

Checklist

• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[utksh1]

Focused change with green checks; approving.