refactor: Derive Error using thiserror crate

[vagmcs]

This PR implements issue #15 by migrating from manual error implementations to using the thiserror crate for improved error handling.

Changes

• Added thiserror = "1.0" dependency to Cargo.toml
• Refactored MDPError in src/mdp/model.rs to use thiserror's #[derive(Error)] macro
• Replaced manual Error and Display trait implementations with #[error] attributes
• Removed unused imports (std::error::Error, std::fmt)

Benefits

• Cleaner, more maintainable error definitions
• Automatic Error and Display trait implementations
• Consistent error message formatting
• Reduced boilerplate code

The existing tests continue to pass, ensuring backward compatibility is maintained.