Add NonParallelizable attribute to tests

[glennawatson]

Pull Request Overview

This PR adds the [NonParallelizable] attribute to UI test classes to prevent test failures caused by concurrent access to static classes. This is a targeted fix to address flaky test behavior in the ReactiveUI test suite.

• Adds [NonParallelizable] attribute to test classes that access static members
• Prevents race conditions during parallel test execution
• Improves test reliability and consistency

[Copilot]

Pull Request Overview

This PR adds the [NonParallelizable] attribute to UI test classes to prevent test failures caused by concurrent access to static classes. This is a targeted fix to address flaky test behavior in the ReactiveUI test suite.

• Adds [NonParallelizable] attribute to test classes that access static members
• Prevents race conditions during parallel test execution
• Improves test reliability and consistency

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.08%. Comparing base (6264b77) to head (1d30bbf).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4161   +/-   ##
=======================================
  Coverage   35.08%   35.08%           
=======================================
  Files         140      140           
  Lines        6016     6016           
  Branches      885      885           
=======================================
  Hits         2111     2111           
  Misses       3743     3743           
  Partials      162      162           

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