In the local (non-gateway) approval path, _resolve_approval_legacy grouped
"once" together with "session" and called approve_session() for both.
approve_session() records the pattern in the session-wide allowlist that the
guard later consults, so a single "Approve once" click silently approved every
later matching tool call for the whole session -- a destructive tool could then
run with no approval card.
Only "session"/"always" persist now. "once" falls through: the
resolve_gateway_approval() call below still unblocks the parked agent thread for
every choice, so the current tool call proceeds, but nothing is written to
_session_approved and the next matching guarded call re-prompts.
Adds tests/test_issue6017_approve_once_no_persist.py, asserting is_approved is
False after "once" and True after "session".
