feat: add TPA pipeline step and social auth disconnect handler

- Add enterprise_associate_by_email pipeline step, replacing the removed pipeline step from platform.
- Add signal handler to unlink enterprise users from an IdP (signal: SAMLAccountDisconnected)
- Inject all enterprise-specific TPA pipeline steps via plugin_settings.

ENT-11566

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>