Add specification tests for jitter

[martincostello]

Add some specification tests for the application of jitter in retries using FsCheck.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (c707b24) to head (f5c2868).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2830   +/-   ##
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  Coverage   96.15%   96.15%           
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  Files         309      309           
  Lines        7123     7123           
  Branches     1005     1005           
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  Hits         6849     6849           
  Misses        221      221           
  Partials       53       53           

Flag	Coverage Δ
linux	96.15% <100.00%> (ø)
macos	96.15% <100.00%> (ø)
windows	96.14% <100.00%> (ø)

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[Copilot]

Pull request overview

This pull request adds property-based specification tests for retry jitter functionality using the FsCheck framework. The changes enable comprehensive testing of jitter algorithms by generating and validating thousands of random test cases.

Key changes:

• Added two FsCheck property-based tests to verify ApplyJitter and DecorrelatedJitterBackoffV2 behavior
• Made two retry helper methods internal to enable direct testing
• Added FsCheck.Xunit package dependency (version 3.3.2)

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
test/Polly.Core.Tests/Retry/RetryHelperTests.cs	Added property-based tests with custom arbitraries for generating test data, and changed helper method visibility
src/Polly.Core/Retry/RetryHelper.cs	Changed two methods from private to internal to enable testing, and reorganized code (moved methods)
test/Polly.Core.Tests/Polly.Core.Tests.csproj	Added FsCheck.Xunit package reference for property-based testing
Directory.Packages.props	Added FsCheck.Xunit version specification (3.3.2)

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