Refactor MonitoredItem.ValueChanged & cert validation for better perf & scalability

[romanett]

Proposed changes

Refactored MonitoredItem.ValueChanged for correctness, performance, and comprehensive test coverage, including improved deadband and array handling. Rewrote equality logic for Variants and arrays. Made certificate validation thread-safe using ConcurrentDictionary and proper semaphore usage. Added extensive unit tests and benchmarks for value change detection.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.