Fix integer overflow in decrypt() and align CLI/docs with constants

[T3pp31]

Summary

This PR addresses three code review findings:

1. Integer overflow in decrypt(): The function now safely handles shift == i16::MIN by widening to i32 before negation
2. CLI default shift: Replaced hardcoded "3" with DEFAULT_SHIFT constant reference
3. Documentation alignment: Updated docstrings to accurately reflect that whitespace-only input is rejected as EmptyText

Key Changes

• src/caesar_cipher.rs:

  • Modified decrypt() to widen shift to i32 before negation, preventing overflow when shift == i16::MIN
  • Normalized the result using rem_euclid() to ensure the value stays within 0..ALPHABET_SIZE
  • Updated docstrings for encrypt_safe() and decrypt_safe() to document that whitespace-only text is rejected
  • Added implementation note explaining the overflow prevention technique

• src/cli.rs:

  • Changed #[arg(short, long, default_value = "3")] to #[arg(short, long, default_value_t = DEFAULT_SHIFT)]
  • Ensures CLI default is derived from the constant, eliminating hardcoded magic numbers

• tests/code_review_fixes_tests.rs (new file):

  • Added 15 comprehensive regression tests covering all three fixes
  • Tests use Given/When/Then comment style for clarity
  • Validates edge cases: i16::MIN, i16::MAX, large negative shifts, and whitespace-only input handling

Implementation Details

The decrypt() fix uses Euclidean remainder to safely normalize the negated shift:

let negated = -(shift as i32);  // Safe: i32 can hold -i16::MIN
let normalized = negated.rem_euclid(ALPHABET_SIZE as i32) as i16;

This ensures the final value is always in 0..26, matching the behavior of the original implementation for all valid inputs while preventing panics on boundary values.

https://claude.ai/code/session_012FR4YMg4EWGZ8Nom1p3Sd4