feat(api): enable SSO auto provision when general registration is disabled

[frenzeldk]

Summary

• Recreation of Enable SSO auto provision when general registration is disabled #1114
• What changed: SSO auto provisiong works even when general registration is disabled
• Why it is correct: This is expected behaviour in SSO context.
• Risk: None

Verification

• Tests run: Default test suite including new and adapted tests for this scenario.
• Manual checks:
  • Registration disabled/SSO Auto Provisioning enabled/SSO Required enabled: Login with new SSO user.
  • Registration disabled/SSO Auto Provisioning enabled/SSO Required disabled: Registration with none-SSO users not possible
  • Registration requires approval/SSO Auto Provisioning enabled/SSO Required enabled: Login with new SSO user - > Requires approval -> login after approval works.

Checklist

• I understand every change in this PR.
• I can explain what it does and why it is correct.
• I disclosed any LLM coding help below.

Screen Recording

Screencast_20260621_204921.webm

LLM Disclosure

• None. Code search and changes done organically by me.

Closes #1112

[Jiralite]

Please add a test for this!