test(scopetimer): cover sink failure boundaries

[stephenlclarke]

Pull Request

Summary

• Cover ScopeTimer sink self-shutdown/restart, queue-boundary, registry-pruning, and secure hard-link rejection paths.
• Raise the main-branch coverage signal above the SonarQube quality-gate threshold without changing production behavior.

Type Of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Maintenance or workflow update

Motivation And Context

The prior coverage-only change improved the main quality gate to 79.5%, still below the required 80%. These deterministic regression tests exercise additional real safety boundaries.

Testing

• Tested locally
• Added or updated tests where appropriate
• Updated documentation where appropriate

Validation: complete local CTest suite (29 tests), header coverage (93.99% lines; 82.01% branches), and diff whitespace validation.

Contributor Checklist

• This pull request is focused on one issue or one coherent change.
• I used Conventional Commits in commit messages and the pull request title.
• I signed my commits with a GitHub-supported signature method.
• I removed credentials, tokens, private keys, personal data, and private environment details from code, tests, logs, and screenshots.