Refocus guide on guarded primary-to-secondary provider fallback

Document same-channel primary/secondary accounts behind one SAS URL, deny-by-default nonacceptance policy, durable operation guards and aggregate deadlines. Remove the per-channel provider split and distinguish new policy-model evidence from prior routing checks.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 9314a109-37df-4021-8e75-bea995d57b23

Author: Hou Chi Chan