fix(python): honor auto_restart on process exit

[haosenwang1018]

Summary

• route Python SDK requests through a stable request proxy so auto_restart is actually honored
• reconnect once and retry the failed RPC when the underlying CLI process exits unexpectedly
• keep existing CopilotSession objects wired to the refreshed transport after reconnect
• add focused unit coverage for both the retrying and non-retrying paths

Testing

• python -m ruff check python/copilot/client.py python/test_client.py
• python -m pytest python/test_client.py -q

Closes #789.