Replaces the six TestUnattendedPlatformExecAskLeak cases in test_approval.py (past its
size cap) with two invariants in a sibling file, both red on main:
- webhook / msgraph_webhook deny from approvals.unattended_mode even with the
TurnRunner-registered notifier, and never reach the human gate;
- api_server denies without a notifier and keeps the ask path once a client bridge
(/v1/runs, streaming chat) registers one.
security.md: unattended_mode now names the api_server exception.
Root cause first identified in #100554 (@fangliquanflq); webhook leg from #134821.
e760a20d89