docs(auth): add documentation about errors code when Email Enumeration Protection is activated

[Lyokone]

Description

Update inline documentation for signInWithEmailAndPassword to reflect that user-not-found and wrong-password error codes are no longer returned on projects with email enumeration protection enabled (the default since September 2023). The invalid-credential code entry is rewritten to explain it replaces both codes. The password-auth doc guide is also updated with a corrected code example.

Related Issues

• Fixes firebase_auth: FirebaseAuthException not caught properly during signInWithEmailAndPassword #17580

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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