Fix errors when title is not specified

Since the optional `title` becomes `title: null` when not provided, the following error occurs.
This occurs with Claude Code 1.0.100.

```console
[DEBUG] MCP server "example": Connection failed after 1591ms: [
  {
    "code": "invalid_type",
    "expected": "string",
    "received": "null",
    "path": [
      "serverInfo",
      "title"
    ],
    "message": "Expected string, received null"
  }
]
```

By adding `compact`, the `title` key will be removed if the optional `title` keyword argument is not specified.

test/mcp/tool_test.rb

@@ -33,6 +33,14 @@ module MCP
       assert_equal({ name: "mock_tool", title: "Mock Tool", description: "a mock tool for testing", inputSchema: { type: "object" } }, tool.to_h)
     end
 
+    test "#to_h does not have `:title` key when title is omitted" do
+      tool = Tool.define(
+        name: "mock_tool",
+        description: "a mock tool for testing",
+      )
+      refute tool.to_h.key?(:title)
+    end
+
     test "#to_h includes annotations when present" do
       tool = TestTool
       expected_annotations = {