.Net: fix: fall back to ToString() when logging function results with unregistered types (#13884)

Fixes #13681

## Problem

When a `KernelFunction` returns a value whose runtime type is not
registered in `AbstractionsJsonContext` (e.g.
`Microsoft.Extensions.AI.TextContent` returned by an MCP tool via
`.AsKernelFunction()`), the function result logging in
AOT/source-generation mode throws `NotSupportedException` during JSON
serialization. The exception is caught but produces an unhelpful log
entry:

```
Function SomePlugin-SomeTool result: Failed to log function result value
System.NotSupportedException: JsonTypeInfo metadata for type 'Microsoft.Extensions.AI.TextContent' was not provided by TypeInfoResolver of type 'Microsoft.SemanticKernel.AbstractionsJsonContext'.
```

## Solution

Add a `ToString()` fallback in the `NotSupportedException` catch block
of `LogFunctionResultValueInternal`. When JSON serialization fails
because the runtime type is absent from the source-generated context,
the logger now calls `resultValue?.Value?.ToString()` to produce useful
output instead of the generic error message.

## Testing

- Added `ItShouldFallBackToToStringWhenJsonSerializationIsNotSupported`
unit test that uses a restricted source-generated
`JsonSerializerContext` (containing only `object` and
`IDictionary<string, object?>`) to simulate the AOT scenario, verifying
that `ToString()` output appears in the log instead of the failure
message.
- All existing `ItShouldLogFunctionResultOfAnyType` test cases are
unchanged.

---------

Co-authored-by: octo-patch <octo-patch@github.com>
Co-authored-by: SergeyMenshykh <68852919+SergeyMenshykh@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: SergeyMenshykh <sergemenshikh@gmail.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 5 people

dotnet/src/SemanticKernel.Abstractions/Functions/KernelFunctionLogMessages.cs

@@ -161,7 +161,22 @@ private static void LogFunctionResultValueInternal(this ILogger logger, string?
             }
             catch (NotSupportedException ex)
             {
-                s_logFunctionResultValue(logger, pluginName, functionName, "Failed to log function result value", ex);
+                // Fall back to ToString() when JSON serialization isn't supported for this type
+                // (e.g. Microsoft.Extensions.AI.TextContent is not registered in AbstractionsJsonContext)
+                try
+                {
+                    var toStringValue = resultValue?.Value?.ToString() ?? string.Empty;
+                    s_logFunctionResultValue(logger, pluginName, functionName, toStringValue, null);
+                }
+                catch (Exception toStringEx)
+                {
+                    s_logFunctionResultValue(
+                        logger,
+                        pluginName,
+                        functionName,
+                        "Failed to log function result value",
+                        new AggregateException(ex, toStringEx));
+                }
             }
         }
     }

dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionLogMessagesTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Microsoft.Extensions.Logging;
 using Microsoft.SemanticKernel;
@@ -10,7 +11,7 @@
 
 namespace SemanticKernel.UnitTests.Functions;
 
-public class KernelFunctionLogMessagesTests
+public partial class KernelFunctionLogMessagesTests
 {
     [Theory]
     [InlineData(typeof(string))]
@@ -48,9 +49,46 @@ public void ItShouldLogFunctionResultOfAnyType(Type resultType)
             It.IsAny<Func<It.IsAnyType, Exception?, string>>()));
     }
 
+    [Fact]
+    public void ItShouldFallBackToToStringWhenJsonSerializationIsNotSupported()
+    {
+        // Arrange
+        var logger = new Mock<ILogger>();
+        logger.Setup(l => l.IsEnabled(It.IsAny<LogLevel>())).Returns(true);
+
+        // TypeNotInJsonContext cannot be cast to string and is not registered in the restricted JSON context
+        var unserializableValue = new TypeNotInJsonContext();
+        var functionResult = new FunctionResult(KernelFunctionFactory.CreateFromMethod(() => { }), unserializableValue);
+
+        // Use a restricted JsonSerializerOptions that knows about object but not TypeNotInJsonContext,
+        // simulating the AOT scenario where AbstractionsJsonContext is used and an unregistered
+        // MEAI type (e.g. Microsoft.Extensions.AI.TextContent) is returned from an MCP tool.
+        var restrictedOptions = RestrictedJsonContext.Default.Options;
+
+        // Act
+        logger.Object.LogFunctionResultValue("p1", "f1", functionResult, restrictedOptions);
+
+        // Assert - ToString() fallback should have been used, not the error message
+        logger.Verify(l => l.Log(
+            LogLevel.Trace,
+            0,
+            It.Is<It.IsAnyType>((o, _) => o.ToString() == "Function p1-f1 result: TypeNotInJsonContext()"),
+            null,
+            It.IsAny<Func<It.IsAnyType, Exception?, string>>()));
+    }
+
     private sealed class User
     {
         [JsonPropertyName("name")]
         public string? Name { get; set; }
     }
+
+    private sealed class TypeNotInJsonContext
+    {
+        public override string ToString() => "TypeNotInJsonContext()";
+    }
+
+    [JsonSerializable(typeof(IDictionary<string, object?>))]
+    [JsonSerializable(typeof(object))]
+    private sealed partial class RestrictedJsonContext : JsonSerializerContext { }
 }