fix: remove unnecessary non-null assertion on fullMessage.key

fullMessage.key is always present on WAMessage. Use consistent style
with stanza-2 handling at line 297.

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

src/Utils/decode-wa-message.ts

@@ -407,7 +407,7 @@ export const decryptMessageNode = (
 							viewOnceInner?.videoMessage?.viewOnce ||
 							viewOnceInner?.audioMessage?.viewOnce
 						) {
-							fullMessage.key!.isViewOnce = true
+							fullMessage.key.isViewOnce = true
 						}
 					} catch (err: any) {
 						// Check if this is a final failure after all retries exhausted