Fix: stop reentrancy loop when reassigning tech and group

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes #45821
• Reassigning a ticket's technician and group at the same time could trigger an infinite loop and a Gateway Timeout.
  The old assign removal is now deferred to the moment the new actor is actually added, instead of running synchronously during the ticket update, which broke the re-entrancy.

Screenshots (if appropriate):