test: cover python delete_session contract

[haosenwang1018]

Summary

• add a Python client test for delete_session()
• verify the client sends the expected session.delete RPC payload
• verify successful deletion also clears the local session cache entry

Why

CopilotClient.delete_session() is part of the public Python API, but it had no focused contract test covering either the request shape or the local _sessions cleanup behavior. This PR locks both in place so regressions are caught in CI.

Validation

• python -m pytest -q python/test_client.py -k 'delete_session_sends_rpc_and_clears_local_cache'
• 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.