fix(auth): log out before Facebook sign-in to clear stale cached token

[demolaf]

Closes #879

When a user switches accounts in the native Facebook app without signing out of FirebaseUI, the Facebook SDK retains a stale cached token. On the next sign-in attempt this caused a silent failure with "User logged in as different Facebook user".

Calling logOut() at the start of signInWithFacebook clears the local token cache before each sign-in, forcing a fresh authentication.

[gemini-code-assist]

Code Review

This pull request adds a logOut() call to the Facebook sign-in flow to prevent stale token issues and includes a unit test for verification. The reviewer identified that the current placement of the logOut() call is likely too late to affect the current sign-in attempt and should be moved to the launcher. Furthermore, there is an inconsistency in the use of the login manager provider that should be addressed to ensure correct behavior when using test overrides.

[russellwheatley]

@demolaf - I've made some updates:

• Wrapped pre-sign-in Facebook logOut() in try/catch so a logout failure cannot strand the user in Loading or block the sign-in launcher.
• Logged that swallowed logout failure as a warning because sign-in still proceeds.
• Added a regression test covering a throwing stale-token logout path, ensuring that specific logout exception is not propagated.
• Updated KDoc for the new loginManagerProvider parameter so its purpose is clear.