fix(tests): update ToolMode conversion tests for conditional ToolChoice behavior

xAI now rejects requests that set ToolChoice when no tools are present in
the request. The production code was already updated to gate ToolChoice on
request.Tools.Count > 0, but the five AsCompletionsRequest_*ToolMode* tests
were still passing an empty tools list and asserting ToolChoice was set.

Changes:
- Add DummyTool() helper that creates a minimal AIFunction for use in tests
- Add Tools = [DummyTool()] to all ToolMode tests so ToolChoice gets set
- Add AsCompletionsRequest_NoTools_DoesNotSetToolChoice to explicitly document
  and verify the new behavior: no tools => no ToolChoice, regardless of ToolMode

Author: kzu

src/xAI.Tests/GrokConversionTests.cs

@@ -285,11 +285,22 @@ static IGrokChatClient CreateClient()
         return mock.Object;
     }
 
+    static AITool DummyTool() => AIFunctionFactory.Create(() => "", "dummy", "A dummy tool");
+
     [Fact]
-    public void AsCompletionsRequest_NullToolMode_SetsAutoToolChoice()
+    public void AsCompletionsRequest_NoTools_DoesNotSetToolChoice()
     {
+        // xAI rejects ToolChoice when no tools are present
         var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = null });
 
+        Assert.Null(request.ToolChoice);
+    }
+
+    [Fact]
+    public void AsCompletionsRequest_NullToolMode_SetsAutoToolChoice()
+    {
+        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = null, Tools = [DummyTool()] });
+
         Assert.NotNull(request.ToolChoice);
         Assert.True(request.ToolChoice.HasMode);
         Assert.Equal(Protocol.ToolMode.Auto, request.ToolChoice.Mode);
@@ -298,7 +309,7 @@ public void AsCompletionsRequest_NullToolMode_SetsAutoToolChoice()
     [Fact]
     public void AsCompletionsRequest_AutoToolMode_SetsAutoToolChoice()
     {
-        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.Auto });
+        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.Auto, Tools = [DummyTool()] });
 
         Assert.NotNull(request.ToolChoice);
         Assert.True(request.ToolChoice.HasMode);
@@ -308,7 +319,7 @@ public void AsCompletionsRequest_AutoToolMode_SetsAutoToolChoice()
     [Fact]
     public void AsCompletionsRequest_NoneToolMode_SetsNoneToolChoice()
     {
-        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.None });
+        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.None, Tools = [DummyTool()] });
 
         Assert.NotNull(request.ToolChoice);
         Assert.True(request.ToolChoice.HasMode);
@@ -318,7 +329,7 @@ public void AsCompletionsRequest_NoneToolMode_SetsNoneToolChoice()
     [Fact]
     public void AsCompletionsRequest_RequireAnyToolMode_SetsRequiredToolChoice()
     {
-        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.RequireAny });
+        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.RequireAny, Tools = [DummyTool()] });
 
         Assert.NotNull(request.ToolChoice);
         Assert.True(request.ToolChoice.HasMode);
@@ -328,7 +339,7 @@ public void AsCompletionsRequest_RequireAnyToolMode_SetsRequiredToolChoice()
     [Fact]
     public void AsCompletionsRequest_RequireSpecificToolMode_SetsFunctionNameToolChoice()
     {
-        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.RequireSpecific("get_weather") });
+        var request = CreateClient().AsCompletionsRequest([], new ChatOptions { ToolMode = ChatToolMode.RequireSpecific("get_weather"), Tools = [DummyTool()] });
 
         Assert.NotNull(request.ToolChoice);
         Assert.True(request.ToolChoice.HasFunctionName);