feat: convert serial console into the serial plugin

plugins/serial/src/backend/ws-message.ts

@@ -0,0 +1,38 @@
+import type { RawData } from "ws";
+
+// Control messages here are tiny (a path, a baud rate) or raw terminal input,
+// bounded by what a user can type or paste, so anything larger is abuse.
+const MAX_MESSAGE_BYTES = 1024 * 1024;
+
+function rawByteLength(raw: RawData): number {
+  if (Buffer.isBuffer(raw)) return raw.length;
+  if (Array.isArray(raw))
+    return raw.reduce((sum, part) => sum + part.length, 0);
+  if (raw instanceof ArrayBuffer) return raw.byteLength;
+  return 0;
+}
+
+/** Parses a WebSocket frame into `{ type, data }`, throwing on anything malformed. */
+export function parseWsMessage(raw: RawData): { type: string; data: unknown } {
+  if (rawByteLength(raw) > MAX_MESSAGE_BYTES) {
+    throw new Error("Message too large");
+  }
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw.toString());
+  } catch {
+    throw new Error("Invalid JSON");
+  }
+
+  if (parsed === null || typeof parsed !== "object" || Array.isArray(parsed)) {
+    throw new Error("Message must be a JSON object");
+  }
+
+  const { type, data } = parsed as { type?: unknown; data?: unknown };
+  if (typeof type !== "string") {
+    throw new Error("Message type must be a string");
+  }
+
+  return { type, data };
+}