fix: do not roll back transaction on partial identity insert error

[amartinj]

Backport of upstream fix ory/kratos#4211 onto our goodnotes branch.

Problem

When CreateIdentities is called with a batch that contains conflicts, Kratos assigns UUIDs to identities and writes them to the accounts DB, but the identities are never persisted in Kratos's own DB. This leaves users in an inconsistent state — they exist in the accounts DB but cannot authenticate.

Root cause: partialErr was returned from inside the Transaction() callback. Since Transaction() rolls back on any non-nil error return, even the successfully-created identities get rolled back and never committed.

Fix

Cherry-pick of upstream commit 82660f04e:

• persistence/sql/identity/persister_identity.go: declare partialErr outside the transaction, return nil from the callback (so successful identities are committed), return partialErr.ErrOrNil() after the transaction.
• identity/manager.go: add NewCreateIdentitiesError() constructor; fix ErrOrNil() to handle nil receiver.

Checklist

• I have added tests that prove my fix is effective or that my feature works.

[noells]

LGTM