Extract Python session I/O modules

[t-kalinowski]

Summary

Extract the remaining Python session stdin/readline and runtime stdio code into focused modules so python_session.rs is centered on session lifecycle, CPython embedding, callback registration, and the public facade.

User-facing changes

No intended behavior changes. The Python backend stdin/readline behavior, raw stdin reads, interrupt handling, prompt visibility, prompt completion, plot gating, callback registration, and IPC messages are preserved.

Internal changes

• Add python_session/state.rs for shared session state, prompt helpers, stdin read accounting, raw stdin errors, and the Unix prompt-wait unit tests.
• Add python_session/windows_stdin.rs for Windows turn input queuing, raw turn input, stdin pipe draining, pending-byte checks, and interrupt turn handling.
• Add python_session/stdio.rs for Python runtime C stdio setup, PTY stdio setup, C line reads, and Python thread release while blocking on stdin.
• Update python_session/unix_stdin.rs to import shared state and stdio helpers from the new modules.