Add health tests for certificates and nb api endpoint resolution

[arudell]

Description

This pull request introduces several enhancements and fixes related to SDN certificate health checks, DNS resolution, and remediation guidance in the SDN diagnostics module. The main improvements include new certificate validation tests, more robust DNS resolution logic, and clearer remediation instructions for detected issues.

Certificate Health Checks:

• Added two new health tests: Test-SdnCertificateExpired (checks for expired/expiring SDN certificates) and Test-SdnCertificateMultiple (detects multiple certificates with the same subject name), including their configuration, documentation links, and integration into server and load balancer health validation flows. [1] [2] [3] [4] [5]
• Updated Get-SdnMuxCertificate and Get-SdnServerCertificate to accept a NetworkControllerOid switch for more flexible certificate retrieval, and improved their documentation. [1] [2] [3]

DNS Resolution Improvements:

• Refactored Test-SdnNetworkControllerApiNameResolution to accept a simple endpoint string, enumerate and filter DNS servers, and attempt resolution against each, with detailed failure reporting and remediation guidance.

Remediation Guidance Enhancements:

• Improved remediation output formatting in Write-HealthValidationInfo to handle array-based instructions and provide clearer, actionable steps for certificate and DNS issues. [1] [2]

Configuration and Impact Updates:

• Updated impact and documentation details for certificate and DNS health tests in SdnDiag.Health.Config.psd1 to reflect new behaviors and provide direct links to troubleshooting guides.

These changes collectively improve the accuracy and clarity of SDN health diagnostics, especially around certificate management and DNS configuration, making troubleshooting more effective for operators and support teams.

Change type

• Bug fix (non-breaking change)
• Code style update (formatting, local variables)
• New Feature (non-breaking change that adds new functionality without impacting existing)
• Breaking change (fix or feature that may cause functionality impact)
• Other

Checklist:

• My code follows the style and contribution guidelines of this project.
• I have tested and validated my code changes.