test: cover python list_sessions missing field

[haosenwang1018]

Summary

• add a focused Python client test for list_sessions() when the server omits the sessions field entirely
• verify the client still sends the expected empty payload
• verify the Python API returns an empty list for the missing-field default case

Why

list_sessions() already had targeted coverage for filtered calls and explicit empty arrays, but not for the server default-path where the sessions field is absent altogether. This PR locks down that fallback behavior so regressions in empty-response handling are caught in CI.

Validation

• python -m pytest -q python/test_client.py -k 'list_sessions_returns_empty_list_when_sessions_field_missing'
• git diff --check

[SteveSandersonMS]

Thanks @haosenwang1018 for submitting all these PRs with test coverage.

As it stands these are not something we would merge because:

1. They only cover Python, and we need to cover all the languages we support. Making the test suites diverge significantly will make it harder to keep the languages in sync.
2. They are largely mock-oriented, which is not the test pattern we're trying to focus on. Using mocks means we're locking down internal implementation details instead of observing the true behavior. For tests that describe interactions with the underlying CLI server, we would prefer to use the E2E test infrastructure. This also helps to ensure we're only testing things that can actually happen in reality.

If you're interested in working with us on test coverage that's wonderful and I'd be happy to discuss more. I'd recommend not submitting large numbers of PRs without coordination though as we are really focused on keeping the project moving and we won't necessarily have capacity to review and merge things that are not on our roadmap.