[6.x] Fix OAuth connections for Eloquent users

[FlxRobole]

Problem

The OAuth provider disconnect/connect method currently throws a BadMethodCallException: Call to undefined method App\Models\User::id() error for all eloquent-managed users:

Both, the cause and fix for this error is quite similar to #10848: The connect and disconnect methods pass the raw App\Models\User model to the vendor code that calls id(), which is only available on Statamic user wrappers.

(see discussion on discourse)

Reproduction

1. Configure Statamic to use the eloquent user repository and an auth provider as per statamic docs (tested with azure & github).
2. Use a standard Eloquent user model (without an id() method, obviously).
3. Sign in using email/password.
4. Open the "Profile > Sign-In providers" page and connect (or disconnect) a provider.

The callback throws the 500 exception.

Fix

• Normalize authenticated models using User::fromUser() before connecting or disconnecting providers.
• Adds some tests that cover connecting, reconnecting, and disconnecting with an Eloquent user (generated by Codex 5.6 Sol Ultra).