Add serialization and edge case tests for Reason property

Co-authored-by: stephentoub <[email protected]>

Author: Copilot

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/Contents/FunctionApprovalResponseContentTests.cs

@@ -35,17 +35,23 @@ public void Constructor_Roundtrips(string id, bool approved)
         Assert.Same(functionCall, content.FunctionCall);
     }
 
-    [Fact]
-    public void Serialization_Roundtrips()
+    [Theory]
+    [InlineData(null)]
+    [InlineData("Custom rejection reason")]
+    public void Serialization_Roundtrips(string? reason)
     {
-        var content = new FunctionApprovalResponseContent("request123", true, new FunctionCallContent("call123", "functionName"));
+        var content = new FunctionApprovalResponseContent("request123", true, new FunctionCallContent("call123", "functionName"))
+        {
+            Reason = reason
+        };
 
         var json = JsonSerializer.Serialize(content, AIJsonUtilities.DefaultOptions);
         var deserializedContent = JsonSerializer.Deserialize<FunctionApprovalResponseContent>(json, AIJsonUtilities.DefaultOptions);
 
         Assert.NotNull(deserializedContent);
         Assert.Equal(content.Id, deserializedContent.Id);
         Assert.Equal(content.Approved, deserializedContent.Approved);
+        Assert.Equal(content.Reason, deserializedContent.Reason);
         Assert.NotNull(deserializedContent.FunctionCall);
         Assert.Equal(content.FunctionCall.CallId, deserializedContent.FunctionCall.CallId);
         Assert.Equal(content.FunctionCall.Name, deserializedContent.FunctionCall.Name);

test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/FunctionInvokingChatClientApprovalsTests.cs

@@ -524,6 +524,66 @@ public async Task MixedApprovalResponsesWithCustomAndDefaultReasonsAsync()
         await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, streamingOutput, expectedDownstreamClientInput);
     }
 
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public async Task RejectedApprovalResponsesWithEmptyOrWhitespaceReasonUsesDefaultMessageAsync(string? reason)
+    {
+        var options = new ChatOptions
+        {
+            Tools =
+            [
+                new ApprovalRequiredAIFunction(AIFunctionFactory.Create(() => "Result 1", "Func1")),
+            ]
+        };
+
+        List<ChatMessage> input =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant,
+            [
+                new FunctionApprovalRequestContent("callId1", new FunctionCallContent("callId1", "Func1")),
+            ]) { MessageId = "resp1" },
+            new ChatMessage(ChatRole.User,
+            [
+                new FunctionApprovalResponseContent("callId1", false, new FunctionCallContent("callId1", "Func1"))
+                {
+                    Reason = reason
+                },
+            ]),
+        ];
+
+        List<ChatMessage> expectedDownstreamClientInput =
+        [
+            new ChatMessage(ChatRole.User, "hello"),
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected.")
+            ]),
+        ];
+
+        List<ChatMessage> downstreamClientOutput =
+        [
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        List<ChatMessage> output =
+        [
+            new ChatMessage(ChatRole.Assistant, [new FunctionCallContent("callId1", "Func1")]),
+            new ChatMessage(ChatRole.Tool,
+            [
+                new FunctionResultContent("callId1", result: "Tool call invocation rejected.")
+            ]),
+            new ChatMessage(ChatRole.Assistant, "world"),
+        ];
+
+        await InvokeAndAssertAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+
+        await InvokeAndAssertStreamingAsync(options, input, downstreamClientOutput, output, expectedDownstreamClientInput);
+    }
+
     [Fact]
     public async Task ApprovedInputsAreExecutedAndFunctionResultsAreConvertedAsync()
     {