Nicer error API

Both the combination of positional argument and naming does not feel
like Ruby.

This change makes `is_error` argument a keyword argument called `error`.

Thanks to this, the usage is more self-explanatory:

```ruby
response = Response.new([], error: true)
```

test/mcp/tool/response_test.rb

@@ -0,0 +1,77 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module MCP
+  class Tool
+    class ResponseTest < ActiveSupport::TestCase
+      test "#initialize with content" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content)
+
+        assert_equal content, response.content
+        refute response.error?
+      end
+
+      test "#initialize with content and error set to true" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content, error: true)
+
+        assert_equal content, response.content
+        assert response.error?
+      end
+
+      test "#initialize with content and error explicitly set to false" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content, error: false)
+
+        assert_equal content, response.content
+        refute response.error?
+      end
+
+      test "#error? for a standard response" do
+        response = Response.new(nil, error: false)
+        refute response.error?
+      end
+
+      test "#error? for an error response" do
+        response = Response.new(nil, error: true)
+        assert response.error?
+      end
+
+      test "#to_h for a standard response" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content)
+        actual = response.to_h
+
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        refute actual[:isError]
+      end
+
+      test "#to_h for an error response" do
+        content = [{
+          type: "text",
+          text: "Unauthorized",
+        }]
+        response = Response.new(content, error: true)
+        actual = response.to_h
+        assert_equal [:content, :isError].sort, actual.keys.sort
+        assert_equal content, actual[:content]
+        assert actual[:isError]
+      end
+    end
+  end
+end