Fix BYOK endpoint URL: accept full chat-completions URLs, not just base URLs

The OpenAI-compatible adapter unconditionally appended /v1/chat/completions to the
configured endpoint, so a BYOK config whose endpoint is the full gateway URL (e.g.
https://host/v1/chat/completions) produced a doubled path and failed. ResolveChatUrl
now uses the URL as-is when it already targets /chat/completions, appends
/chat/completions to a base ending in /v1, and otherwise appends /v1/chat/completions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Modules/TeamUp.Modules.Integrations/Ai/ModelClients.cs

@@ -50,8 +50,7 @@ internal sealed class OpenAiCompatibleModelClient(HttpClient http) : IModelClien
         var stopwatch = Stopwatch.StartNew();
         try
         {
-            var baseUrl = (request.Endpoint ?? "https://api.openai.com").TrimEnd('/');
-            using var message = new HttpRequestMessage(HttpMethod.Post, $"{baseUrl}/v1/chat/completions");
+            using var message = new HttpRequestMessage(HttpMethod.Post, ResolveChatUrl(request.Endpoint));
             if (!string.IsNullOrEmpty(request.ApiKey))
             {
                 message.Headers.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", request.ApiKey);
@@ -110,6 +109,24 @@ internal sealed class OpenAiCompatibleModelClient(HttpClient http) : IModelClien
         }
     }
 
+    /// <summary>
+    /// Resolve the chat-completions URL from a BYOK endpoint. Accepts a base URL (we append the path),
+    /// a base already ending in <c>/v1</c>, or the full <c>…/chat/completions</c> URL pasted as-is — so
+    /// a user who enters the complete gateway URL doesn't get a doubled path.
+    /// </summary>
+    private static string ResolveChatUrl(string? endpoint)
+    {
+        var url = (endpoint ?? "https://api.openai.com").Trim().TrimEnd('/');
+        if (url.Contains("/chat/completions", StringComparison.OrdinalIgnoreCase))
+        {
+            return url;
+        }
+
+        return url.EndsWith("/v1", StringComparison.OrdinalIgnoreCase)
+            ? $"{url}/chat/completions"
+            : $"{url}/v1/chat/completions";
+    }
+
     private static object[] BuildMessages(ModelRequest request)
     {
         if (request.Messages is not { Count: > 0 })