update names for the message list and object wrapper

[wrenj]

should be clearer

[gemini-code-assist]

Code Review

This pull request refactors the message list schemas by changing the root type from an object containing a 'messages' property to a raw array. To maintain backward compatibility, new wrapper schemas have been introduced to preserve the original object structure. I have no further feedback, though please note that the schema changes are breaking for direct consumers, and the new files should be registered in the test suite's configuration as suggested by the reviewer.