feat(core-eslint): rule no-direct-socket-io

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

packages/core-eslint/rules/no-direct-socket-io.test.js

@@ -0,0 +1,30 @@
+// packages/core-eslint/rules/no-direct-socket-io.test.js
+import { RuleTester } from "eslint";
+import rule from "./no-direct-socket-io.js";
+
+const tester = new RuleTester({
+  languageOptions: { ecmaVersion: 2022, sourceType: "module" },
+});
+
+tester.run("no-direct-socket-io", rule, {
+  valid: [
+    // Allowed inside core-realtime
+    { code: 'import { Server } from "socket.io";', filename: "/repo/packages/core-realtime/src/socket-io-realtime-server.ts" },
+    // Allowed in app servers
+    { code: 'import { Server } from "socket.io";', filename: "/repo/apps/web-next/server.ts" },
+    // Allowed elsewhere when not importing socket.io
+    { code: 'import { foo } from "bar";', filename: "/repo/packages/blog/src/foo.ts" },
+  ],
+  invalid: [
+    {
+      code: 'import { Server } from "socket.io";',
+      filename: "/repo/packages/blog/src/foo.ts",
+      errors: [{ messageId: "noDirectSocketIO" }],
+    },
+    {
+      code: 'import { io } from "socket.io-client";',
+      filename: "/repo/packages/blog/src/ui/Component.tsx",
+      errors: [{ messageId: "noDirectSocketIOClient" }],
+    },
+  ],
+});