Remove special-casing of string enumerables in McpServerTool (modelcontextprotocol#699)

We special-case string enumerables, translating them to an array of text content blocks, but other enumerables just get serialized, and there's a reasonable expectation that returning a string[] would produce a JSON array of strings. Just delete the special-casing.

Author: stephentoub

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -278,12 +278,6 @@ public override async ValueTask<CallToolResult> InvokeAsync(
                 StructuredContent = structuredContent,
             },
 
-            IEnumerable<string> texts => new()
-            {
-                Content = [.. texts.Select(x => new TextContentBlock { Text = x ?? string.Empty })],
-                StructuredContent = structuredContent,
-            },
-            
             IEnumerable<AIContent> contentItems => ConvertAIContentEnumerableToCallToolResult(contentItems, structuredContent),
 
             IEnumerable<ContentBlock> contents => new()

tests/ModelContextProtocol.Tests/Configuration/McpServerBuilderExtensionsToolsTests.cs

@@ -254,8 +254,7 @@ public async Task Can_Call_Registered_Tool_With_Array_Result()
 
         Assert.NotNull(result.Content);
         Assert.NotEmpty(result.Content);
-        Assert.Equal("hello Peter", (result.Content[0] as TextContentBlock)?.Text);
-        Assert.Equal("hello2 Peter", (result.Content[1] as TextContentBlock)?.Text);
+        Assert.Equal("""["hello Peter","hello2 Peter"]""", (result.Content[0] as TextContentBlock)?.Text);
 
         result = await client.CallToolAsync(
             "SecondCustomTool",

tests/ModelContextProtocol.Tests/Server/McpServerToolTests.cs

@@ -356,9 +356,8 @@ public async Task CanReturnCollectionOfStrings()
         var result = await tool.InvokeAsync(
             new RequestContext<CallToolRequestParams>(mockServer.Object),
             TestContext.Current.CancellationToken);
-        Assert.Equal(2, result.Content.Count);
-        Assert.Equal("42", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
-        Assert.Equal("43", Assert.IsType<TextContentBlock>(result.Content[1]).Text);
+        Assert.Single(result.Content);
+        Assert.Equal("""["42","43"]""", Assert.IsType<TextContentBlock>(result.Content[0]).Text);
     }
 
     [Fact]