feat: implemented alerting for expiring certificates for application gateway

[nc-shahidazim]

Description

This PR enables SSL certificate‑expiry alerting for the Application Gateway by integrating the existing application-insights-availability-test Terraform module from the dtos-devops-templates repository. The module is now configured to run availability tests against the Application Gateway endpoint and surface SSL‑related failures through Application Insights.

Details

• Introduced usage of the application-insights-availability-test module from dtos-devops-templates.
• Configured the module with a health check endpoint to validate the Application Gateway certificate.
• Enabled alerting based on SSL validation failures reported by the availability test.
• All underlying functionality comes from PR #276 in the templates repository.

Context

The platform previously lacked automated monitoring for expiring TLS certificates on the Application Gateway. With the enhancements delivered in PR #276 of the templates repository, we can now consume this functionality directly through a standardised module. This PR activates and configures that feature for our environment.

Pre‑requisites Prior to Release

Before applying this change to any environment, ensure:

1. Health Check Target URL

   • A valid health check endpoint must be deployed and reachable.
   • The availability test uses this URL to validate the Application Gateway certificate.

2. Azure DevOps Pipeline Variable

   • The environment variable OCP_APIM_SUBSCRIPTION_KEY must be defined.
   • Required for authenticating requests to the health check endpoint.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[github-actions]

Unit Test Results

✔️ Tests 975 / 975 - passed in 66.5s
📝 Coverage 47.07%
📏 4756 / 10412 lines covered 🌿 1167 / 2172 branches covered
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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud