Replace assert_empty with assert_equal using explicit literals

Due to overly aggressive detection by RuboCop Minitest, the distinction between `[]` and `{}` was uniformly replaced with `assert_empty`,
making it impossible to differentiate between them.
As a result of https://github.com/rubocop/rubocop-minitest/issues/344, where the `Minitest/AssertEmptyLiteral` cop was disabled,
the return value has been expressed using explicit literal forms.

test/mcp/tool/response_test.rb

@@ -107,7 +107,7 @@ module MCP
         actual = response.to_h
 
         assert_equal [:content, :isError, :structuredContent], actual.keys
-        assert_empty actual[:content]
+        assert_equal([], actual[:content])
         assert_equal structured_content, actual[:structuredContent]
         refute actual[:isError]
       end
@@ -118,7 +118,7 @@ module MCP
         actual = response.to_h
 
         assert_equal [:content, :isError, :structuredContent], actual.keys
-        assert_empty actual[:content]
+        assert_equal([], actual[:content])
         assert_equal structured_content, actual[:structuredContent]
         refute actual[:isError]
       end