fix: persist user ID across before-user-created hook for external providers (#41309)

[abhinavlevi]

What is the current behavior?

When users sign up via external providers (OAuth/SSO), the before-user-created hook payload receives a generated user.id that is subsequently discarded and re-generated during account creation.

As a result, the UUID sent in the webhook payload differs from the id stored in auth.users.

Closes supabase/supabase#41309

What is the new behavior?

The candidate user.ID generated prior to invoking the before-user-created hook is now persisted in UserProvidedData and reused during user creation in PostgreSQL. The webhook payload user.id and auth.users.id are now guaranteed to match for all OAuth/SSO providers.

Additional context

Problem Breakdown

1. triggerBeforeUserCreatedExternal (internal/api/hooks.go) calls params.ToUserModel(...), which generates a UUID for the candidate user model and sends it in the webhook payload.
2. createAccountFromExternalIdentity (internal/api/external.go) called params.ToUserModel(...) a second time under case models.CreateAccount:, overwriting user.ID with a second UUID before saving to PostgreSQL.

Solution

1. Added UserID uuid.UUID to UserProvidedData (internal/api/provider/provider.go).
2. Captured userData.UserID = user.ID in triggerBeforeUserCreatedExternal.
3. Reused userData.UserID in createAccountFromExternalIdentity if userData.UserID != uuid.Nil.

Verification

Ran the integration test suite locally against PostgreSQL:

go test -v ./internal/api -run "TestExternal|TestSignup"