fix: address Cubic round-4 P2 issues

- isTodo: allow optional id to match Todo interface, preventing
  todos without ids from being silently dropped
- messageHasContentFromSDK: treat unknown part types as empty
  (continue) instead of content (return true) for parity with
  existing storage logic
- readMessagesFromSDK in recover-empty-content-message-sdk: wrap
  SDK call in try/catch to prevent recovery from throwing

src/features/background-agent/session-todo-checker.ts

@@ -4,7 +4,7 @@ function isTodo(value: unknown): value is Todo {
   if (typeof value !== "object" || value === null) return false
   const todo = value as Record<string, unknown>
   return (
-    typeof todo["id"] === "string" &&
+    (typeof todo["id"] === "string" || todo["id"] === undefined) &&
     typeof todo["content"] === "string" &&
     typeof todo["status"] === "string" &&
     typeof todo["priority"] === "string"