Fix Ctrl+C handling for interactive commands launched from Python

[bradhe]

Summary

• restore default SIGINT behavior in the Python CLI entrypoint before invoking the native Rust command runner
• this makes Ctrl+C terminate interactive command flows consistently when invoked via tower Python script
• add a unit test that verifies the entrypoint installs SIG_DFL for SIGINT and still forwards argv to the native runner

Notes

• this change is in the Python shim layer and applies broadly to interactive waits (for example login polling and other long-running interactive flows)

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[konstantinoscs]

If we know it's os-portable, I'm good

[konstantinoscs]

Does this work accross OSs?