feat: replace Nito.AsyncEx with SemaphoreSlim and sign assemblies

[bingenito]

Replace Nito.AsyncEx with SemaphoreSlim and implement a custom AsyncCountdownEvent for strong-name support. Sign assemblies and update InternalsVisibleTo attributes to include public keys.

[codecov]

Codecov Report

❌ Patch coverage is 61.68224% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.7%. Comparing base (2841b93) to head (c0eb7db).

Files with missing lines	Patch %	Lines
...g/dotnet/src/Core/Threading/AsyncCountdownEvent.cs	51.7%	33 Missing and 8 partials ⚠️

❌ Your patch check has failed because the patch coverage (61.6%) is below the target coverage (100.0%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1075     +/-   ##
=======================================
- Coverage   65.8%   65.7%   -0.1%     
=======================================
  Files        337     338      +1     
  Lines       9989   10086     +97     
  Branches    1154    1224     +70     
=======================================
+ Hits        6573    6629     +56     
- Misses      3151    3182     +31     
- Partials     265     275     +10     

Files with missing lines	Coverage Δ
...ng/dotnet/src/Client/Client/MessageRouterClient.cs	83.3% <100.0%> (+0.3%)	⬆️
...Instrumentation/MessageRouterDiagnosticObserver.cs	78.0% <ø> (ø)
...g/dotnet/src/Core/Threading/AsyncCountdownEvent.cs	51.7% <51.7%> (ø)

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[lilla28]

[bingenito]

Thanks for confirming compliance @lilla28. There were a few things like this I was unsure of so had left in draft.