xunit v3 preparation

[martincostello]

Cherry-pick changes from #2440 to prepare to switch to xunit v3.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.12%. Comparing base (b7321af) to head (812d090).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2723   +/-   ##
=======================================
  Coverage   96.12%   96.12%           
=======================================
  Files         309      309           
  Lines        7118     7118           
  Branches     1008     1008           
=======================================
  Hits         6842     6842           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.12% <100.00%> (ø)
macos	96.12% <100.00%> (ø)
windows	96.11% <100.00%> (ø)

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[Copilot]

Pull Request Overview

This PR prepares the codebase for xunit v3 by introducing a shared TestCancellation utility and standardizing test cancellation token usage across the entire test suite. The changes improve test consistency and prepare for potential future xunit compatibility requirements.

Key changes:

• Addition of TestCancellation.Token helper to centralize test cancellation token management
• Systematic replacement of CancellationToken.None with TestCancellation.Token in all test files
• Addition of thread-safe semaphore release logic in production code to prevent ObjectDisposedException

Reviewed Changes

Copilot reviewed 128 out of 128 changed files in this pull request and generated no comments.

File	Description
test/Shared/TestCancellation.cs	New shared utility providing centralized test cancellation token
Multiple test project .csproj files	Added references to shared TestCancellation utility
100+ test files	Replaced CancellationToken.None with TestCancellation.Token throughout
src/Polly/Bulkhead/BulkheadEngine.cs	Added safe semaphore release to prevent ObjectDisposedException

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