vmalertmanager: remove prefix from empty subroute receiver

[AndrewChubatiuk]

fixes #2185

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Summary by cubic

Stop prefixing empty subroute receiver names in Alertmanager config generation. This restores receiver inheritance and prevents invalid receiver names in nested routes.

• Bug Fixes
  • Skip CR prefixing when receiver name is empty in buildCRPrefixedName (fixes AlertmanagerConfig conversion fails if matcher has not receiver defined #2185).
  • Added tests for subroute inheritance and updated the changelog.

^{Written for commit 69df595. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}