Remove localhost restriction from LDS registration certificate validation

[Copilot]

Proposed changes

The RegistrationValidator_CertificateValidation method prevented servers from registering with remote LDS/LDS-ME servers by requiring the LDS certificate domain to resolve to the local hostname's IP address.

Changes:

• Removed RegistrationValidator_CertificateValidation method and its event handler registration
• LDS registration now uses the standard CertificateValidator which validates certificate chain, expiration, trust stores, and revocation lists
• Pass configuration.ApplicationUri to UpdateAsync instead of null to ensure application certificates are loaded correctly even if the subject name is changed

This maintains security through proper certificate validation while enabling remote LDS registration.

Related Issues

• Fixes Registering to the Remote LDS Server is not possible #1967

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

Further comments

Testing: 156/156 server tests and 7836/7836 core tests passed. No new security issues detected.

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[romanett]

@copilot please take applicationUri from configuration instead of passing null, to ensure appliction certificates are loaded also if the subject name is changed

[Copilot]

Done in 54e3217. Changed from applicationUri: null to configuration.ApplicationUri to ensure application certificates are loaded correctly even if the subject name is changed.