Add MSW infrastructure: handlers, fixtures, request-log, and Vitest setup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mocks/node.js

@@ -0,0 +1,8 @@
+/**
+ * MSW server for Node.js (Vitest).
+ * @module mocks/node
+ */
+import { setupServer } from "msw/node";
+import { handlers } from "./handlers/index.js";
+
+export const server = setupServer(...handlers);