Remove unused block parameters

There are some unused block variables. This change removes block parameters
that consist solely of unused block parameters.
Where `Hash#values` suffices instead of ignoring unused variables,
replace such code with `Hash#values`.

lib/mcp/server.rb

@@ -253,7 +253,7 @@ module MCP
     end
 
     def list_tools(request)
-      @tools.map { |_, tool| tool.to_h }
+      @tools.values.map(&:to_h)
     end
 
     def call_tool(request)
@@ -293,7 +293,7 @@ module MCP
     end
 
     def list_prompts(request)
-      @prompts.map { |_, prompt| prompt.to_h }
+      @prompts.values.map(&:to_h)
     end
 
     def get_prompt(request)