fix(ringcentral-admin): consistent preview-then-confirm on AI extension write

create_ai_scheduling_extension previously checked approval_text before the
dry_run branch, so the required text couldn't be discovered via the tool.
Move the approval_text check below dry_run and return the expected text in
the dry-run response, matching the other write tools. Drop approval_text
from the required schema so dry_run can be called to obtain it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>