[MCP] Updated icon theme description and added validation (#8996)

Author: glen-84

src/HotChocolate/Adapters/src/Adapters.Mcp.Core/Properties/McpAdapterResources.Designer.cs

@@ -45,6 +45,9 @@
   <data name="OperationToolIcon_InvalidIconSourceScheme" xml:space="preserve">
     <value>The icon source URI must use the HTTP, HTTPS, or data scheme.</value>
   </data>
+  <data name="OperationToolIcon_InvalidIconTheme" xml:space="preserve">
+    <value>The icon theme must be 'light' or 'dark'.</value>
+  </data>
   <data name="ReadResourceHandler_ResourceNotFound" xml:space="preserve">
     <value>Resource not found.</value>
   </data>

src/HotChocolate/Adapters/src/Adapters.Mcp.Core/Properties/McpAdapterResources.resx

@@ -84,10 +84,24 @@ public IList<string>? Sizes
     }
 
     /// <summary>
-    /// The optional theme for this icon. Can be "light", "dark", or a custom theme identifier. Used
-    /// to specify which UI theme the icon is designed for.
+    /// The optional theme for this icon. Can be "light" or "dark". Used to specify which UI theme
+    /// the icon is designed for.
     /// </summary>
-    public string? Theme { get; init; }
+    public string? Theme
+    {
+        get;
+        init
+        {
+            if (value is not null and not "light" and not "dark")
+            {
+                throw new ArgumentException(
+                    OperationToolIcon_InvalidIconTheme,
+                    nameof(Theme));
+            }
+
+            field = value;
+        }
+    }
 
     [GeneratedRegex(@"^([0-9]+x[0-9]+|any)\z")]
     private static partial Regex IconSizeRegex();

src/HotChocolate/Adapters/src/Adapters.Mcp.Core/Storage/OperationToolIcon.cs

@@ -115,4 +115,43 @@ public void OperationToolIcon_InvalidSizes_ThrowsArgumentException(params string
             + "(Parameter 'Sizes')",
             exception.Message);
     }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("light")]
+    [InlineData("dark")]
+    public void OperationToolIcon_ValidTheme_Succeeds(string? theme)
+    {
+        // arrange & act
+        var exception =
+            Record.Exception(
+                () => new OperationToolIcon(new Uri("https://example.com/icon.png"))
+                {
+                    Theme = theme
+                });
+
+        // assert
+        Assert.Null(exception);
+    }
+
+    [Theory]
+    [InlineData("blue")]
+    [InlineData("LIGHT")]
+    [InlineData("Dark")]
+    public void OperationToolIcon_InvalidTheme_ThrowsArgumentException(string theme)
+    {
+        // arrange & act
+        var exception =
+            Record.Exception(
+                () => new OperationToolIcon(new Uri("https://example.com/icon.png"))
+                {
+                    Theme = theme
+                });
+
+        // assert
+        Assert.IsType<ArgumentException>(exception);
+        Assert.Equal(
+            "The icon theme must be 'light' or 'dark'. (Parameter 'Theme')",
+            exception.Message);
+    }
 }