Keep the original casing of model name for custom endpoint to enable Foundry Local (#413)

This PR contains the following 2 fixes:
1. Keep the original casing of the model's name for custom endpoint configuration.
2. Fix the error reporting to handle exception messages that already contains newline at the end.

OpenAI models require lower case for their names, but the custom endpoint may be case-sensitive, such as the endpoint set up by Foundry Local. So, we should keep the original casing for custom endpoint configurations.

Author: daxian-dbw

shell/AIShell.Kernel/Shell.cs

@@ -694,7 +694,12 @@ internal async Task RunREPLAsync()
                     while (ex.InnerException is { })
                     {
                         sb ??= new(message, capacity: message.Length * 3);
-                        sb.Append($"\n   Inner -> {ex.InnerException.Message}");
+                        if (sb[^1] is not '\n')
+                        {
+                            sb.Append('\n');
+                        }
+
+                        sb.Append($"   Inner -> {ex.InnerException.Message}");
                         ex = ex.InnerException;
                     }
 
@@ -703,8 +708,9 @@ internal async Task RunREPLAsync()
                         message = sb.ToString();
                     }
 
+                    string separator = message.EndsWith('\n') ? "\n" : "\n\n";
                     Host.WriteErrorLine()
-                        .WriteErrorLine($"Agent failed to generate a response: {message}\n\n{stackTrace}")
+                        .WriteErrorLine($"Agent failed to generate a response: {message}{separator}{stackTrace}")
                         .WriteErrorLine();
                 }
             }

shell/agents/AIShell.OpenAI.Agent/GPT.cs

@@ -55,13 +55,16 @@ public GPT(
         Description = description;
         Endpoint = endpoint?.Trim().TrimEnd('/');
         Deployment = deployment;
-        ModelName = modelName.ToLowerInvariant();
         SystemPrompt = systemPrompt;
         Key = key;
         AuthType = authType;
 
         Dirty = false;
-        ModelInfo = ModelInfo.TryResolve(ModelName, out var model) ? model : null;
+        ModelInfo = ModelInfo.TryResolve(modelName, out var model) ? model : null;
+
+        // OpenAI models require the model name to be lower case. e.g., 'gpt-4.1' works but not 'GPT-4.1'.
+        // However, custom endpoint may be case-sensitive on the model name, such as Foundry Local.
+        ModelName = ModelInfo is { } ? modelName.ToLowerInvariant() : modelName;
 
         bool noEndpoint = string.IsNullOrEmpty(Endpoint);
         bool noDeployment = string.IsNullOrEmpty(Deployment);