Return state from ProcessCodeFlowAsync (#248)

[AndreyVMarkelov]

Fixes #248.

The Uri overload of ProcessCodeFlowAsync validated the state parameter but never returned it, so OAuth2Response.State was always null. Thread state through to the inner overload and into the response.

Added a test that reproduces the null state and passes with the fix.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.30%. Comparing base (c25d774) to head (da8c0f5).

Files with missing lines	Patch %	Lines
...pbox-sdk-dotnet/Dropbox.Api/DropboxOauth2Helper.cs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
+ Coverage   54.09%   58.30%   +4.20%     
==========================================
  Files          16       16              
  Lines        1403     1403              
  Branches      134      134              
==========================================
+ Hits          759      818      +59     
+ Misses        604      524      -80     
- Partials       40       61      +21     

Flag	Coverage Δ
integration	54.81% <66.66%> (+4.20%)	⬆️
unit	10.54% <0.00%> (ø)

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