Use NullabilityInfoContext to determine dictionary value nullability

[patrikwlund]

A follow-up of #3022 (issue) and #3023 (PR).

The #3023 PR broke the behavior for many of our IReadOnlyDictionary<string, string> (and similar) members.

The reason is that the logic for reading and interpreting NullableAttribute and NullableContextAttribute flags is too simplified and is incorrect for several cases. It just happens to pass the current test cases because of the layout of the test type and its members.

Too simplified

For example, it ignores the optimization of the flags array into a single byte if they are all the same value. An optimized Dictionary<string, string> has a single [1] flag, not two or three flags as the current logic expects.

It also does a lot of assumptions related to reference vs value types. An IDictionary itself can be a value type, as can the key, both of which would affect the length of the flag array. It should be checking the number of value types and the number of flags together.

It also doesn't seem to fully respect the NullableContextAttribute that's placed on the type containing the member. And the tests did not account for differences in the context value, as the testing type always only has a single flag. So the other flag value is not tested.

My suggested change

For NET 6 and onwards we should rely on the official NullabilityInfoContext to tell us what is and isn't nullable. For earlier versions we can still have custom logic, which we can try to improve on if we want to put in the effort. But it will require a lot of effort to make it fully compliant and flexible.

The tests I added do not pass with the custom flag logic, but they do pass with the NullabilityInfoContext code.

The tests could be more flexible or enumerated in other more dynamic ways if you would like, to get the number of lines down, but I tried to follow the existing pattern of laying them all out in-line.

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Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.46%. Comparing base (7a7230d) to head (bf5c554).

Files	Patch %	Lines
...SwaggerGen/SchemaGenerator/MemberInfoExtensions.cs	54.54%	5 Missing ⚠️

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@@            Coverage Diff             @@
##           master    #3041      +/-   ##
==========================================
- Coverage   90.59%   90.46%   -0.13%     
==========================================
  Files          74       74              
  Lines        2988     2980       -8     
  Branches      478      470       -8     
==========================================
- Hits         2707     2696      -11     
- Misses        281      284       +3     

Flag	Coverage Δ
Linux	90.46% <54.54%> (-0.13%)	⬇️
Windows	90.46% <54.54%> (-0.13%)	⬇️
macOS	90.46% <54.54%> (-0.13%)	⬇️

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[martincostello]

Thanks for the PR! Just a few questions but otherwise this looks great.

[martincostello]

Thanks for fixing this - the fix is now available in NuGet as version 6.7.2.