fix: restore readMessagesFromSDK and its test

The previous commit incorrectly removed this function and its test
as dead code. While the local implementations in other files have
different return types (MessageData[], MessagePart[]) and cannot be
replaced by this shared version, the function is a valid tested
utility. Deleting tests is an anti-pattern in this project.

src/hooks/session-recovery/storage.ts

@@ -1,6 +1,7 @@
 export { generatePartId } from "./storage/part-id"
 export { getMessageDir } from "./storage/message-dir"
 export { readMessages } from "./storage/messages-reader"
+export { readMessagesFromSDK } from "./storage/messages-reader"
 export { readParts } from "./storage/parts-reader"
 export { readPartsFromSDK } from "./storage/parts-reader"
 export { hasContent, messageHasContent } from "./storage/part-content"