test(scopetimer): cover custom sink callback guards

[stephenlclarke]

Pull Request

Summary

• Add regression coverage for quiescent custom-sink configuration, recursive custom-sink flushing, reserved output space, and large thread identifiers.
• Resolve the SonarQube review: all intentional C++20 style and sink-lifecycle findings are recorded as accepted, while the coverage gate is addressed with executable tests.

Type Of Change

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

Motivation And Context

The main-branch scan identified a 79.2% new-code coverage result, just below the 80% quality-gate threshold. The added tests exercise previously uncovered behavior without changing the intentional production implementation.

Testing

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

Validation: complete local CTest suite (29 tests), header coverage (92.62% lines; 80.58% 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.