Refactor MCP tool and server to handle optional server_context parameter

- Updated `call` method in `MCP::Tool` and `MCP::Prompt` to accept `server_context` as an optional parameter.
- Refactored `MCP::Server` to streamline argument handling for tool and prompt calls, introducing helper methods `call_tool_with_args` and `call_prompt_template_with_args`.
- Adjusted tests to reflect changes in method signatures and ensure compatibility with optional `server_context` parameter.

lib/mcp/prompt.rb

@@ -9,7 +9,7 @@ module MCP
       attr_reader :description_value
       attr_reader :arguments_value
 
-      def template(args, server_context:)
+      def template(args, server_context: nil)
         raise NotImplementedError, "Subclasses must implement template"
       end
 
@@ -57,7 +57,7 @@ module MCP
           prompt_name name
           description description
           arguments arguments
-          define_singleton_method(:template) do |args, server_context:|
+          define_singleton_method(:template) do |args, server_context: nil|
             instance_exec(args, server_context:, &block)
           end
         end