Fix flaky test

[martincostello]

Fix flaky test if there's a race in the shared pool between parallel test executions.

[Copilot]

🟡 Changes recommended

The FakeTimeProvider assertion will fail deterministically on .NET 8+ until corrected.

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Pull request overview

This pull request isolates cancellation-token pool tests from the shared process-wide pool to prevent race-related flakiness.

Changes:

• Uses a dedicated pool for reusable-pool tests.
• Exposes the pooled implementation internally for testing.
• The FakeTimeProvider reuse assertion still needs correction.

File summaries

File	Summary
test/Polly.Core.Tests/Utils/CancellationTokenSourcePoolTests.cs	Uses an isolated pool but contains an incorrect reuse assertion on pooled targets.
src/Polly.Core/Utils/CancellationTokenSourcePool.Pooled.cs	Enables internal construction of the pooled implementation for tests.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.02%. Comparing base (5f9fa58) to head (8a1dbab).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3231   +/-   ##
=======================================
  Coverage   98.02%   98.02%           
=======================================
  Files         321      321           
  Lines        7099     7099           
  Branches     1089     1089           
=======================================
  Hits         6959     6959           
  Misses         98       98           
  Partials       42       42           

Flag	Coverage Δ
linux	97.97% <ø> (-0.05%)	⬇️
macos	98.01% <ø> (ø)
windows	98.02% <ø> (ø)

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