fix: narrow GetGroupInfoAsync exception scope to KeyNotFoundException only

Broad catch (Exception) in GetGroupInfoAsync was swallowing all errors
(ObjectDisposedException, InvalidOperationException, etc.) and returning
null, causing JoinGroupAsync to silently fall through to "create new group"
on non-not-found errors. Changed to catch (KeyNotFoundException) only and
removed the LogError call on the not-found path (it is the expected hot path
when joining a new group, not an error condition).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rbenzing

LibEmiddle/API/LibEmiddleClient.Group.cs

@@ -288,11 +288,11 @@ public async Task<bool> LeaveGroupAsync(string groupId)
             LoggingManager.LogDebug(nameof(LibEmiddleClient), $"Retrieved group info for {groupId}");
             return groupSession;
         }
-        catch (Exception ex)
+        catch (KeyNotFoundException)
         {
-            LoggingManager.LogError(nameof(LibEmiddleClient),
-                $"Failed to get group info for {groupId}: {ex.Message}");
+            // Group not found — expected path when joining a new group, not an error
             return null;
         }
+        // All other exceptions (ObjectDisposedException, InvalidOperationException, etc.) propagate
     }
 }