fix(telegram): improve error messages for 403 bot not member errors

- Detect 403 'bot is not a member' errors specifically
- Provide actionable guidance for users to fix the issue
- Fixes #48273 where outbound sendMessage fails with 403

Root cause:
When a Telegram bot tries to send a message to a channel/group it's not
a member of, the API returns 403 'bot is not a member of the channel chat'.
The error message was not clear about how to fix this.

Fix:
1. Detect 403 errors in wrapTelegramChatNotFoundError
2. Provide clear error message explaining the issue
3. Suggest adding the bot to the channel/group

extensions/telegram/src/send.ts

@@ -491,7 +491,20 @@ function createTelegramRequestWithDiag(params: {
 }
 
 function wrapTelegramChatNotFoundError(err: unknown, params: { chatId: string; input: string }) {
-  if (!CHAT_NOT_FOUND_RE.test(formatErrorMessage(err))) {
+  const errorMsg = formatErrorMessage(err);
+  
+  // Check for 403 "bot is not a member" error
+  if (/403.*bot.*not.*member|bot.*blocked/i.test(errorMsg)) {
+    return new Error(
+      [
+        `Telegram send failed: bot is not a member of the chat (chat_id=${params.chatId}).`,
+        "Fix: Add the bot to the channel/group, or ensure it has not been removed/blocked.",
+        `Input was: ${JSON.stringify(params.input)}.`,
+      ].join(" "),
+    );
+  }
+  
+  if (!CHAT_NOT_FOUND_RE.test(errorMsg)) {
     return err;
   }
   return new Error(