Add ToolAnnotations support (#124)

* Add ToolAnnotations support

* Address feedback

src/ModelContextProtocol/Server/McpServerToolAttribute.cs

@@ -1,18 +1,73 @@
 namespace ModelContextProtocol.Server;
 
 /// <summary>
-/// Used to mark a public method as an MCP tool.
+/// Used to indicate that a method should be considered an MCP tool and describe it.
 /// </summary>
 [AttributeUsage(AttributeTargets.Method)]
 public sealed class McpServerToolAttribute : Attribute
 {
-    /// <summary>Gets the name of the tool.</summary>
-    /// <remarks>If <see langword="null"/>, the method name will be used.</remarks>
-    public string? Name { get; }
+    // Defaults based on the spec
+    private const bool DestructiveDefault = true;
+    private const bool IdempotentDefault = false;
+    private const bool OpenWorldDefault = true;
+    private const bool ReadOnlyDefault = false;
+
+    // Nullable backing fields so we can distinguish
+    internal bool? _destructive;
+    internal bool? _idempotent;
+    internal bool? _openWorld;
+    internal bool? _readOnly;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="McpServerToolTypeAttribute"/> class.
     /// </summary>
-    /// <param name="name">The name of the tool. If <see langword="null"/>, the method name will be used.</param>
-    public McpServerToolAttribute(string? name = null) => Name = name;
+    public McpServerToolAttribute()
+    {
+    }
+
+    /// <summary>Gets the name of the tool.</summary>
+    /// <remarks>If <see langword="null"/>, the method name will be used.</remarks>
+    public string? Name { get; set; }
+
+    /// <summary>
+    /// Gets or sets a human-readable title for the tool.
+    /// </summary>
+    public string? Title { get; set; }
+
+    /// <summary>
+    /// Gets or sets whether the tool may perform destructive updates to its environment.
+    /// </summary>
+    public bool Destructive 
+    {
+        get => _destructive ?? DestructiveDefault; 
+        set => _destructive = value; 
+    }
+
+    /// <summary>
+    /// Gets or sets whether calling the tool repeatedly with the same arguments will have no additional effect on its environment.
+    /// </summary>
+    public bool Idempotent 
+    {
+        get => _idempotent ?? IdempotentDefault;
+        set => _idempotent = value; 
+    }
+
+    /// <summary>
+    /// Gets or sets whether this tool may interact with an "open world" of external entities
+    /// (e.g. the world of a web search tool is open, whereas that of a memory tool is not).
+    /// </summary>
+    public bool OpenWorld
+    {
+        get => _openWorld ?? OpenWorldDefault; 
+        set => _openWorld = value; 
+    }
+
+    /// <summary>
+    /// Gets or sets whether the tool does not modify its environment.
+    /// </summary>
+    public bool ReadOnly 
+    {
+        get => _readOnly ?? ReadOnlyDefault; 
+        set => _readOnly = value; 
+    }
 }