fix: code strategy should not reset UI messages when populating login flow (PS-557)

[splaunov]

This PR fixes an issue when duplicate credentials error added by OIDC strategy into new login flow is lost because code strategy clears all messages when populating login flow.

This is where OIDC strategy adds duplicate credentials error into flow ui messages list:

kratos/selfservice/strategy/oidc/strategy.go

Lines 581 to 590 in 234b6f2

	if validationErr := new(schema.ValidationError); errors.As(err, &validationErr) {
	for _, m := range validationErr.Messages {
	m := m
	rf.UI.Messages.Add(&m)
	}
	} else {
	rf.UI.Messages.Add(text.NewErrorValidationDuplicateCredentialsOnOIDCLink())
	}
	lf, err := s.registrationToLogin(ctx, w, r, rf)

Related issue(s)

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• I have referenced an issue containing the design document if my change
  introduces a new feature.
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• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

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