fix: Ignore the Chain-of-Thought in AI response (#952)

- Improve chat response processing to handle thinking patterns using regular expressions.
- Migrate server value by removing trailing '/chat/completions' path.

src/Models/OpenAI.cs

@@ -19,7 +19,15 @@ namespace SourceGit.Models
         public string Server
         {
             get => _server;
-            set => SetProperty(ref _server, value);
+            set
+            {
+                // migrate old server value
+                if (!string.IsNullOrEmpty(value) && value.EndsWith("/chat/completions", StringComparison.Ordinal))
+                {
+                    value = value.Substring(0, value.Length - "/chat/completions".Length);
+                }
+                SetProperty(ref _server, value);
+            }
         }
 
         public string ApiKey