Refactor CLI args and add input validation with shift range support

[T3pp31]

Summary

This PR refactors the CLI argument handling to reduce code duplication, adds comprehensive input size validation, expands shift value range support, and improves error messages for better user experience.

Key Changes

CLI Refactoring

• Introduced CipherArgs struct to consolidate common arguments between Encrypt and Decrypt commands, eliminating ~40 lines of duplicated field definitions
• Updated command variants to use Encrypt(CipherArgs) and Decrypt(CipherArgs) tuple struct syntax
• Simplified command matching logic in run_cli()

Input Validation

• Added MAX_INPUT_SIZE constant (1MB) to prevent processing excessively large inputs
• Implemented file size validation before reading to avoid memory issues
• Added input size checks for text arguments and interactive mode input
• Enhanced error messages to be more descriptive (e.g., "Input text cannot be empty or whitespace-only")

Shift Value Handling

• Added MIN_SHIFT constant (-25) to support negative shift values
• Introduced validate_shift_input() function with comprehensive validation:
  • Returns parsed shift value with optional warning message
  • Warns when shift is outside typical range (-25 to 25) but still accepts it
  • Gracefully handles invalid input by returning default shift
• Updated prompt_for_shift() to use new validation function and display more helpful prompts
• Updated brute force to include shift 0 (showing original text for reference)

Whitespace Handling

• Enhanced empty text validation to reject whitespace-only input
• Updated error messages across encrypt_safe and decrypt_safe functions
• Added comprehensive test coverage for whitespace edge cases

Documentation & Tests

• Updated help text descriptions to reflect new shift range support
• Added 20+ new tests covering:
  • Input size limit validation
  • Shift value validation with boundary cases
  • Whitespace-only input rejection
  • Leading/trailing whitespace preservation
  • Brute force shift 0 inclusion
• Updated existing tests to match new error messages

Minor Improvements

• Optimized main() to use env::args().count() instead of collecting into vector
• Updated version to 1.0.4
• Improved code organization with better comments and test grouping

https://claude.ai/code/session_01UwAwi9MgGMEYuzf5NqHPuF