test: Increase test coverage (#2367)

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## Overview

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Tests**
- Added new tests covering notification handling, validation methods,
configuration defaults, and error scenarios.
- Expanded coverage on concurrency safety, cache management, and utility
function edge cases.
- Improved test organization by replacing a single complex test with
multiple focused and granular tests.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: Manav-Aggarwal