Add missing mutations

[martincostello]

• Add tests for the remaining undetected mutations.
• Remove remaining explicit doNothing delegate allocations.
• Increase mutation score for Polly to 100.
• Increase minimum code coverage for Polly.Specs to ~90%.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.09%. Comparing base (9190943) to head (b1b3c4c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Polly/Retry/AsyncRetryTResultSyntax.cs	85.71%	1 Missing ⚠️
src/Polly/Retry/RetrySyntax.cs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2508      +/-   ##
==========================================
+ Coverage   90.76%   94.09%   +3.32%     
==========================================
  Files         311      311              
  Lines        7484     7431      -53     
  Branches     1070     1043      -27     
==========================================
+ Hits         6793     6992     +199     
+ Misses        543      342     -201     
+ Partials      148       97      -51     

Flag	Coverage Δ
linux	94.09% <95.23%> (+3.32%)	⬆️
macos	94.09% <95.23%> (+3.32%)	⬆️
windows	94.07% <95.23%> (+3.33%)	⬆️

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