PIN prompt with fido2 security key Reauth challenge

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- [x] I determined this is the correct repository in which to report this bug.

### Summary of the issue

**Context**
I have a Google account with [session-length controls enabled](https://knowledge.workspace.google.com/admin/security/set-session-length-for-google-cloud-services) which requires security key reauth.
After configuring a PIN on my security key (optional) gcloud reauth prompts prompt for PIN entry due to [user_verification=preferred](https://github.com/googleapis/google-cloud-python/commit/a474872eb58f7d4c8301527b8721a2461bb4438a) being set in the webauthn request handler ([legacy repo issue link](https://github.com/googleapis/google-auth-library-python/pull/1798) with [helpful clarification of uv and up](https://github.com/googleapis/google-auth-library-python/pull/1798#issuecomment-3723817079)).

**Expected Behavior:**
No PIN entry required.

**Actual Behavior:**
PIN entry is required to complete the reauth challenge.

### API client name and version

Google Cloud SDK 578.0.0

### Reproduction steps: code

n/a

### Reproduction steps: supporting files

n/a

### Reproduction steps: actual results

n/a

### Reproduction steps: expected results

n/a

### OS & version + platform

MacOS 15.7.7

### Python environment

google-cloud-sdk bundled python

### Python dependencies

n/a

### Additional context

_No response_