Remove special-casing of string enumerables in McpServerTool (#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.

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -270,12 +270,6 @@ internal sealed partial class AIFunctionMcpServerTool : McpServerTool
                 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()