Fix mattermost channel selection (robusta-dev#1125)

* fix mattermost channel selection

* indentation fix

src/robusta/integrations/mattermost/client.py

@@ -105,7 +105,12 @@ def get_channel_id(self, channel_name: str) -> Optional[str]:
             response = response.json()
             if not len(response):
                 return None
-            return response[0].get("id")
+            # Search can return mulitple channels, check the name
+            for chan in response:
+                if chan["name"] == channel_name:
+                    return chan["id"]
+        else:
+            logging.error("Received error response from MM on channel search: {str(response)}")
 
     def get_team_id(self, team_name: str) -> Optional[str]:
         if not self.is_admin: