Expect no id in the refusal of an unreadable request id

## Motivation and Context

The null id test of `test/mcp/server_test.rb` (#612) expects the refusal to carry `"id": null`, and the test
of an id that is neither a string nor an integer (#613) only checks that `response[:id]` is nil. #615,
merged after both, leaves the `id` member out of an error response whose request id cannot be read,
so the first test fails on main (CI run 39142457575) and the second no longer pins the shape of the response.
Both now expect a response without an `id` member; the library is unchanged.

## How Has This Been Tested?

`test/mcp/server_test.rb`; against main the null id test fails as in CI.

Author: koic

test/mcp/server_test.rb

@@ -1942,7 +1942,7 @@ def read_resource_request(uri)
 
       response = @server.handle({ jsonrpc: "2.0", id: nil, method: "tools/call", params: { name: "count", arguments: {} } })
 
-      assert_equal({ jsonrpc: "2.0", id: nil, error: { code: -32600, message: "Invalid Request", data: "Request ID must not be null" } }, response)
+      assert_equal({ jsonrpc: "2.0", error: { code: -32600, message: "Invalid Request", data: "Request ID must not be null" } }, response)
       assert_equal 0, calls
     end
 
@@ -1996,7 +1996,7 @@ def read_resource_request(uri)
       [3.14, true, false, { a: 1 }, [1]].each do |id|
         response = @server.handle({ jsonrpc: "2.0", id: id, method: "ping" })
 
-        assert_nil response[:id], id.inspect
+        refute response.key?(:id), id.inspect
         assert_equal(-32600, response[:error][:code], id.inspect)
       end
     end