Fix system sleep prevention and add comprehensive test suite

- Fixed terminal control preventing system sleep by improving rustyline configuration and adding proper cleanup
- Added signal handling for graceful termination and terminal state reset
- Implemented comprehensive test suite with 58 unit and integration tests
- Added testing dependencies: tempfile, mockall, tokio-test, serial_test
- Created proper Drop implementation for InputHandler to ensure terminal cleanup
- Enhanced exit handling in both normal exit and /exit command

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -23,3 +23,10 @@ syntect = "5.1"
 regex = "1.0"
 futures = "0.3"
 tokio-stream = "0.1"
+signal-hook = "0.3"
+
+[dev-dependencies]
+tempfile = "3.0"
+mockall = "0.12"
+tokio-test = "0.4"
+serial_test = "3.0"