fix: surface real errors when adding user to Mattermost team

_ensure_team_member silently swallowed every exception from
add_user_to_team, assuming failure meant the user was already a
member. Real failures (bad permissions, domain restrictions, wrong
team ID) were masked and only surfaced later as a confusing "No team
member found" error when adding the user to a channel. Now checks
actual membership via get_team_member and raises a clear
MattermostError on genuine add failures.

apps/chat/integrations/mattermost/client.py

@@ -72,12 +72,22 @@ class MattermostClient:
     def _ensure_team_member(self, mm_user_id: str) -> None:
         if not self._team_id:
             return
+        try:
+            self._driver.teams.get_team_member(self._team_id, mm_user_id)
+            return  # already a member
+        except ResourceNotFound:
+            pass
+        except Exception as exc:
+            raise MattermostError(f"Team membership lookup failed: {exc}") from exc
+
         try:
             self._driver.teams.add_user_to_team(
                 self._team_id, options={"team_id": self._team_id, "user_id": mm_user_id}
             )
-        except Exception:
-            pass  # already a member or team not configured — not fatal
+        except Exception as exc:
+            raise MattermostError(
+                f"Adding user {mm_user_id} to team failed: {exc}"
+            ) from exc
 
     def get_or_create_user(self, user_id: UUID) -> str:
         """