fix: default python session.log level to info

[haosenwang1018]

Summary

• make Python CopilotSession.log() send level="info" when callers omit the level argument
• add a focused client test that locks the default request shape in place

Why

Fixes #810. The public Python API documents level as defaulting to "info", but the current implementation omits level from the RPC entirely when the caller does not pass it. This PR aligns the implementation with the documented/default API contract.

Validation

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

[SteveSandersonMS]

Thanks @haosenwang1018 but I'd rather not make this change because:

1. It makes no difference to the overall behavior - the underlying runtime will apply info as a default anyway, so the behavior is already correct
2. This change would make the Python SDK diverge from others, also without benefit