refactor(plugins): decouple bundled plugin runtime loading

test/helpers/plugins/plugin-routing.ts

@@ -0,0 +1,7 @@
+export {
+  __testing as sessionBindingTesting,
+  registerSessionBindingAdapter,
+} from "../../../src/infra/outbound/session-binding-service.js";
+export { setActivePluginRegistry } from "../../../src/plugins/runtime.js";
+export { resolveAgentRoute } from "../../../src/routing/resolve-route.js";
+export { createTestRegistry } from "../../../src/test-utils/channel-plugins.js";