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)
```

lib/mcp/tool/response.rb

@@ -3,15 +3,26 @@
 module MCP
   class Tool
     class Response
-      attr_reader :content, :is_error
+      NOT_GIVEN = Object.new.freeze
+
+      attr_reader :content
+
+      def initialize(content, deprecated_error = NOT_GIVEN, error: false)
+        if deprecated_error != NOT_GIVEN
+          warn("Passing `error` with the 2nd argument of `Response.new` is deprecated. Use keyword argument like `Response.new(content, error: error)` instead.", uplevel: 1)
+          error = deprecated_error
+        end
 
-      def initialize(content, is_error = false)
         @content = content
-        @is_error = is_error
+        @error = error
+      end
+
+      def error?
+        !!@error
       end
 
       def to_h
-        { content:, isError: is_error }.compact
+        { content:, isError: error? }.compact
       end
     end
   end