fix(channels): check response status in send() for Telegram, Slack, and Discord

Reliability fix: check HTTP response status in channel send methods

src/channels/telegram.rs

@@ -376,12 +376,22 @@ impl Channel for TelegramChannel {
             "parse_mode": "Markdown"
         });
 
-        self.client
+        let resp = self
+            .client
             .post(self.api_url("sendMessage"))
             .json(&body)
             .send()
             .await?;
 
+        if !resp.status().is_success() {
+            let status = resp.status();
+            let err = resp
+                .text()
+                .await
+                .unwrap_or_else(|e| format!("<failed to read response body: {e}>"));
+            anyhow::bail!("Telegram sendMessage failed ({status}): {err}");
+        }
+
         Ok(())
     }