Fix exit codes, shell escaping, validation, and logging

[google-labs-jules]

This PR addresses several critical issues and improvements identified in docs/CODE_REVIEW.md:

1. Exit Code Retrieval (#5): Implemented logic to retrieve the actual exit code (using echo $? or equivalent) for commands that complete during background monitoring after a timeout. This ensures exit_code is accurate instead of defaulting to 0 or None.
2. Shell Escaping Fix (#15): Replaced manual character escaping in write_file (sudo mode) with base64 encoding/decoding. This prevents vulnerabilities and bugs related to special characters (newlines, quotes, etc.) in file content.
3. Enable Mode Validation (#9): Added validation in _execute_enable_mode_command_internal to check if the session is actually in enable mode (prompt ends with #) before executing commands. It attempts to re-enter enable mode if the validation fails.
4. Logging Improvements (#14): Implemented rate limiting for debug logs inside tight recv loops in command_executor.py to prevent log flooding during long operations or idle states.

Verified with targeted reproduction tests and existing regression tests.

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PR created automatically by Jules for task 11506711411728051988 started by @devnullvoid

[google-labs-jules]

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