fix: prevent prisma crash on OIDC accounts with non-standard fields

[mikimandoki]

Description

Kanidm includes an issued_token_type field in its token response which isn't in the Prisma schema, causing a DB crash before the account could be created. Rather than adding another provider-specific case to linkAccount, this PR replaces the entire chain with a generic sanitisation pass using Prisma.AccountScalarFieldEnum as the source of truth. Any provider with non-standard fields now works without further changes.

Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me

See #682