Auth: providerData entries carry the Firebase UID and shared profile instead of each provider's own identity #77

From the Copilot review of #73 (#73 (comment)), on the credential sign-in from #69 by @TomislavMladenov.

FirebaseUserImpl.providerData is rebuilt from providerIds as UserInfo(displayName, email, phoneNumber, photoUrl, provider, uid), so every provider entry reports the Firebase account UID and the shared profile. On Android each entry carries the provider's own subject and profile (a Google or Apple subject generally differs from FirebaseUser.uid), so code that keys on providerData[i].uid sees the wrong identity, including after linking.

Suggested fix: preserve the providerUserInfo records from the IdP and account-lookup responses, persist them with the user, and merge them on link instead of reconstructing entries from providerIds.