test: the on-demand routes' other callers ack over the socket, not the mailbox

test_api_v3_bool_coercion, test_api_v3_lazy_plugin_discovery and
test_web_api's stop test checked the mailbox write; they now get an ack
from a mocked control socket. The stop route touches no manager any more,
so it leaves the removed-catch-all sample. A request that was sent is
never 'not listening' (pins the sent check the mutation run let through).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/web_interface/test_api_v3_unhandled_errors.py

@@ -81,7 +81,6 @@ REMOVED_CATCH_ALLS = [
     ("GET", "/api/v3/config/main", None),
     ("GET", "/api/v3/config/secrets", None),
     ("GET", "/api/v3/display/modes", None),
-    ("POST", "/api/v3/display/on-demand/stop", {}),
     ("GET", "/api/v3/cache/list", None),
     ("GET", "/api/v3/plugins/installed", None),
     ("GET", "/api/v3/plugins/health", None),