fix(nextcloud-talk): calculate signature over message text, not JSON body

NextCloud Talk's Bot API expects the HMAC signature to be calculated
over `random + message` (just the message text), not over the entire
JSON body `random + {"message":"..."}`.

This fix changes the signature calculation to use the plain message
text while keeping the HTTP request body as JSON.

Fixes authentication errors when bots try to send responses:
"Nextcloud Talk: authentication failed - check bot secret"

Closes #4073

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

extensions/nextcloud-talk/src/send.ts

@@ -90,7 +90,7 @@ export async function sendMessageNextcloudTalk(
   const bodyStr = JSON.stringify(body);
 
   const { random, signature } = generateNextcloudTalkSignature({
-    body: bodyStr,
+    body: message,
     secret,
   });