Document Hash Integrity Validator

[mftee]

Implement a HashValidator module that enforces strict validation rules on document hashes before they are submitted to or queried from the Stellar blockchain.

Requirements:

• Create a HashValidator struct with the following methods:
  • validate_sha256(hash: &str) -> Result<(), ValidationError> — ensures the hash is a 64-character lowercase hex string
  • validate_sha512(hash: &str) -> Result<(), ValidationError> — ensures 128-character lowercase hex
  • normalize(hash: &str) -> String — lowercases and trims whitespace
  • detect_algorithm(hash: &str) -> Option<HashAlgorithm> — returns SHA256, SHA512, or None based on length
• Create a ValidationError enum:

  pub enum ValidationError {
      WrongLength { expected: usize, actual: usize },
      InvalidCharacter { position: usize, character: char },
      EmptyHash,
  }

• Apply HashValidator::validate_sha256 to all incoming document_hash fields in /verify, /submit, /revoke, and /transfer endpoints, returning 400 with a descriptive error if validation fails

Acceptance Criteria:

• A 63-char hash fails with WrongLength
• A hash containing uppercase letters passes after normalization
• A hash with a non-hex character fails with InvalidCharacter specifying the position
• Comprehensive unit tests covering all error variants
• detect_algorithm correctly identifies SHA-256 vs SHA-512 hashes