fix(message-transport): only ignore messages that lack jsonrpc: "2.0" (#448)

Tighten the parse-failure handler: silently ignore postMessage frames
where event.data?.jsonrpc !== "2.0" (i.e. not JSON-RPC at all — null,
undefined, a primitive, or an object from the host environment that
doesn't carry the jsonrpc field). Messages that do carry jsonrpc: "2.0"
but are otherwise malformed still surface via onerror so genuine protocol
errors aren't swallowed.

https://claude.ai/code/session_01DaJpUtSJf4hA7tnu3nJz8s

Co-authored-by: Claude <noreply@anthropic.com>

src/message-transport.ts

@@ -82,7 +82,17 @@ export class PostMessageTransport implements Transport {
       if (parsed.success) {
         console.debug("Parsed message", parsed.data);
         this.onmessage?.(parsed.data);
+      } else if (event.data?.jsonrpc !== "2.0") {
+        // Not a JSON-RPC message at all (e.g. internal frames injected by
+        // the host environment). Ignore silently so the transport stays alive.
+        console.debug(
+          "Ignoring non-JSON-RPC message",
+          parsed.error.message,
+          event,
+        );
       } else {
+        // Has jsonrpc: "2.0" but is otherwise malformed — surface as a real
+        // protocol error.
         console.error("Failed to parse message", parsed.error.message, event);
         this.onerror?.(
           new Error(