Address PR feedback

Author: stephentoub

src/ModelContextProtocol.Analyzers/Diagnostics.cs

@@ -19,4 +19,13 @@ internal static class Diagnostics
         defaultSeverity: DiagnosticSeverity.Warning,
         isEnabledByDefault: true,
         description: "The XML documentation comment contains invalid XML and cannot be processed to generate Description attributes.");
+
+    public static DiagnosticDescriptor McpMethodMustBePartial { get; } = new(
+        id: "MCP002",
+        title: "MCP method must be partial to generate [Description] attributes",
+        messageFormat: "Method '{0}' has XML documentation that could be used to generate [Description] attributes, but the method is not declared as partial.",
+        category: "mcp",
+        defaultSeverity: DiagnosticSeverity.Warning,
+        isEnabledByDefault: true,
+        description: "Methods with MCP attributes should be declared as partial to allow the source generator to emit Description attributes from XML documentation comments.");
 }