desktop: log and recover empty or failed token fetches

[trungutt]

Requests through the Docker AI Gateway fail with failed to get Docker Desktop token for Gateway. Is Docker Desktop running and are you signed in? whenever the token fetched from Docker Desktop comes back empty — yet this case leaves no trace anywhere: the fetch error was discarded, and unlike the expired-token case there was no log line and no recovery attempt.

GetToken()
  ├─ token EXPIRED → logged + forced refresh   (already handled)
  └─ token EMPTY / fetch error → silence       (this PR)

An empty result typically means Desktop's session exists but its read-time token refresh just failed (network blip, revoked refresh token) — the user is signed in, so the error message is misleading and there is nothing in the logs to attribute it to.

This PR makes the empty/failed fetch path behave like the expired path:

• log why the token is unusable (fetch error, empty, or expired — with a safe fingerprint)
• run the same forced refresh, but only when Desktop reports the user as signed in, so signed-out users don't pay the refresh polling budget on every request
• surface HTTP error statuses from the backend client instead of masking them as JSON decode errors