re-enable signing validation

[nohwnd]

Once issues on arcade side is solved, re-enable validation to make sure everytyhing is correct. This has no impact on how we sign the code, just ensures that we signed correctly #15592

[nohwnd]

Triage Analysis 🔍

The current state of signing validation in azure-pipelines-official.yml:

- template: eng/common/templates-official/post-build/post-build.yml@self
  parameters:
    # Temporarily skip validation until infra issues are resolved, based on advice from Arcade.
    enableSigningValidation: false

The arcade SDK has since been updated from the version in use when this was disabled (March 2026) to Microsoft.DotNet.Arcade.Sdk 11.0.0-beta.26310.1 (current in global.json). This is a significant version bump.

Suggested action: Try re-enabling enableSigningValidation: true in azure-pipelines-official.yml and see if a CI build succeeds. If the arcade infrastructure issues have been resolved, validation should pass. If it still fails, the arcade SDK team should be contacted for status.

The one-line change to attempt is:

-        enableSigningValidation: false
+        enableSigningValidation: true

🔍 Triaged by Issue Repro Triage & Auto-Fix 🔍

🔍 Triaged by Issue Repro Triage & Auto-Fix 🔍