The from_mcp_integration test invoked bundle.handler directly as a
function, but HandlerRegistration.handler is a HandlerKind enum
(Once/Stream), not a callable. This caused E0618 (expected function,
found HandlerKind) and a downstream E0282 on the response match arm.
Pattern-match HandlerKind::Once to extract the inner Handler before
calling, mirroring the pattern used in protocol/connection.rs and
from_openapi.rs tests.