CLI-6: Fix Keyboard Interruption Handling

[szmania]

Summary

This PR implements proper keyboard interruption handling for LLM completions and IO operations.

Changes Made

1. Interruptible LLM Completion (cecli/coders/base_coder.py)

• Added interrupt_event asyncio.Event() to track interruptions
• Implemented proper cancellation of LLM completion tasks when interrupted using asyncio.wait() with FIRST_COMPLETED
• Added proper cleanup for cancelled tasks to prevent resource leaks
• Ensures clean KeyboardInterrupt handling without partial responses or garbage errors

2. IO Interruption Propagation (cecli/io.py)

• Added mechanism to propagate interruption events to the coder
• Ensures that keyboard interrupts are properly handled at the IO level
• Maintains prompt session state during interruptions

Problem Solved

Before this change, keyboard interrupts could leave the system in an inconsistent state, potentially causing:

• Partial LLM responses being processed
• Resource leaks from incomplete async operations
• Garbage error messages
• Inconsistent prompt session state

Testing

The changes have been tested with:

• Manual keyboard interruption during LLM completions
• Verification of clean state after interruption
• Confirmation that prompt sessions maintain consistency

Related Issue

Fixes CLI-6: Improve interruption handling to prevent garbage errors and partial responses.