Fix: Clean up database tokens on logout

[evanliu048]

Issue #, if available:

Description of changes:
Fixed two issues with authentication token management:

• Logout not cleaning database: Tokens stored in the database (auth_kv table) were not being deleted during logout, even though SecretStore credentials were properly cleared.
• Null token being written: When using social login, a "null" string was being written to builderid token in the database because the code didn't check if BuilderIdToken actually existed before serializing it.

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