Improve code coverage by skipping unreachable code

[bkoelman]

Replace occurrences of:

throw new UnreachableException();

which gets flagged as non-covered by tests, with an extension method that is marked with [ExcludeFromCodeCoverage].
It works similarly to Debug.Assert, taking a boolean condition.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 98.65772% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.75%. Comparing base (fb78af3) to head (45696f6).
Report is 3 commits behind head on openapi.

Files with missing lines	Patch %	Lines
...e.OpenApi.Swashbuckle/OpenApiEndpointConvention.cs	96.55%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1705      +/-   ##
===========================================
+ Coverage    90.41%   90.75%   +0.34%     
===========================================
  Files          466      466              
  Lines        14434    14451      +17     
  Branches      2283     2293      +10     
===========================================
+ Hits         13050    13115      +65     
+ Misses         933      909      -24     
+ Partials       451      427      -24     

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