Add test coverage for the userinfo profile-fetch fallback

The userinfo request in GIDSignIn.m does not have any test coverage (this is the fallback that fetches basic profile data if the ID token doesn't carry it).

Suggested changes:
- Set up a sign in whose ID token lacks profile claims, so the profile-fetch fallback runs
- Assert the userinfo request URL: scheme, host, path /oauth2/v3/userinfo, and the access_token query parameter
- Include a token containing URL-reserved characters (&, =, #). (testDisconnectNoCallback_tokenWithReservedCharacters does this to test recent changes that ensure those characters are encoded correctly)
- Assert the returned data maps correctly onto GIDProfileData

Start with GIDSignIn.m and the existing testDisconnectNoCallback_tokenWithReservedCharacters test for setup and URL-encoding behavior. Add coverage for an ID token without profile claims, verify the /oauth2/v3/userinfo URL and encoded access_token, and confirm the response maps correctly onto GIDProfileData.

Vom Indexierungsmodell aus dem Issue-Text verfasst.