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.

[SteveSandersonMS]

Thanks for propsing this.

However, this feature has never actually worked across any of the languages - there's non-implemented options for it on Python and Go, and then on TypeScript it tries to restart in certain cases but there are many cases where it won't work.

The feature dates back to the original spike of this SDK. I think we'd be better off removing it until we can make a more solid implementation, and of course only in reaction to real-world evidence of the need that can provide a clear case for what specifically should be supported.

I appreciate the PR you've added here makes it a little closer to working on Python, but it still won't work in a lot of cases (e.g., the CLI subprocess becomes frozen, or it was launched externally in the first place).