Fix OAuthPKCE example browser launch on .NET Core (#334)

[AndreyVMarkelov]

Fixes #334.

Process.Start(url) throws on .NET Core because UseShellExecute defaults to false. Pass a ProcessStartInfo with UseShellExecute = true so the URL opens in the default browser.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.81%. Comparing base (cde11d5) to head (01ba290).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #380   +/-   ##
=======================================
  Coverage   53.81%   53.81%           
=======================================
  Files          16       16           
  Lines        1403     1403           
  Branches      134      134           
=======================================
  Hits          755      755           
  Misses        608      608           
  Partials       40       40           

Flag	Coverage Δ
integration	50.32% <ø> (ø)
unit	10.54% <ø> (ø)

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