Extract Unix Python stdin protocol

[t-kalinowski]

Summary

Extracts the Unix-specific Python stdin and PTY turn-input protocol from python_session.rs into python_session/unix_stdin.rs.

This makes the CPython callback facade easier to inspect while keeping the Unix stdin mechanics in one focused module.

User Facing Changes

No user-facing behavior changes are intended.

Internal Changes

• Moves Unix turn input normalization, PTY feed demand handling, stdin discard after interrupts, protocol failure handling, and stdin read accounting into unix_stdin.
• Keeps python_session.rs responsible for CPython callbacks, runtime setup, and cross-platform branching.
• Leaves Windows turn-input handling unchanged.

Additional Notes

The split is intended to preserve existing worker protocol messages, PTY sequencing, prompt emission, and stdin-wait behavior.