[Infra] Fix new code analysis warnings

[martincostello]

Changes

Fix new code analysis warnings raised when using the .NET 11 SDK in #3867.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.62%. Comparing base (d08b817) to head (d354e42).
⚠️ Report is 19 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4379      +/-   ##
==========================================
- Coverage   75.65%   75.62%   -0.03%     
==========================================
  Files         456      456              
  Lines       18246    18246              
==========================================
- Hits        13804    13799       -5     
- Misses       4442     4447       +5     

Flag	Coverage Δ
unittests-Instrumentation.EventCounters	77.67% <ø> (ø)
unittests-OpAmp.Client	83.95% <ø> (-0.52%)	⬇️

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see 2 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR addresses new C#/.NET code analysis warnings introduced when building the repository with the .NET 11 SDK (per #3867), by making small, analyzer-friendly updates in affected unit tests.

Changes:

• Refactor exception-type checks in WsTransportTest to use modern pattern matching (is ... or ...) instead of chained ||.
• Remove an unnecessary async modifier from a test method that contains no await, eliminating “async method lacks await” warnings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/OpenTelemetry.OpAmp.Client.Tests/WsTransportTest.cs	Updates exception assertions to use is null or ... pattern matching to satisfy analyzers.
test/OpenTelemetry.Instrumentation.EventCounters.Tests/EventCountersMetricsTests.cs	Removes async from a test that performs no awaits to avoid analyzer warnings.

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