Fix webauthn CLI verification always rendering success after credential verification

[Hemang360]

When authenticating via Webauthn during gem push, the browser would show "Failed to fetch" even though authentication succeeded and the gem was pushed. This happened because the server redirected the browser to localhost:PORT to deliver the otp, and if that redirect failed, the browser showed an error, despite the gem CLI already having the otp via polling.

Fix by removing the localhost redirect entirely. The server now always returns "success" after Webauthn verification (the same path safari already used), and the gem CLI relies on polling to receive the otp, which is already how the safari flow worked.

Fixes #4317

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.47%. Comparing base (dd8f309) to head (c5fd6bf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6365      +/-   ##
==========================================
- Coverage   97.07%   94.47%   -2.61%     
==========================================
  Files         489      489              
  Lines       10373    10426      +53     
==========================================
- Hits        10070     9850     -220     
- Misses        303      576     +273     

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