Throw AuthException for revoked or invalid refresh token (#232)

[AndreyVMarkelov]

Fixes #232.

RefreshAccessToken only mapped invalid_grant to an AuthException on a 401 with an exact-string body. A revoked or invalid refresh token actually returns 400 with a JSON invalid_grant body, so it surfaced as a generic HttpRequestException with no useful detail.

• Detect invalid_grant by parsing the OAuth error JSON on any non-success status (400 or 401).
• Throw AuthException carrying the error_description and the real HTTP status code.
• Add AuthException/StructuredException constructors that accept a message and status code.
• Add a live test asserting the exception type, a non-JSON message, and StatusCode 400.

Verified against the live token endpoint (reproduced the generic exception before the fix).

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.38%. Comparing base (2ab8af7) to head (0336631).

Files with missing lines	Patch %	Lines
...ox-sdk-dotnet/Dropbox.Api/DropboxRequestHandler.cs	68.75%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
+ Coverage   58.54%   59.38%   +0.84%     
==========================================
  Files          16       16              
  Lines        1411     1433      +22     
  Branches      135      136       +1     
==========================================
+ Hits          826      851      +25     
+ Misses        524      520       -4     
- Partials       61       62       +1     

Flag	Coverage Δ
integration	55.96% <80.00%> (+0.89%)	⬆️
unit	10.32% <0.00%> (-0.17%)	⬇️

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