[TomikeDS]

Description

Closes #14

Enforces SHA-256 as the only accepted hash algorithm at every service boundary and provides a tested conversion path from user-provided hex strings to the BytesN<32> type required by the Soroban contract.

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Related Issues

Closes #14

Changes Made

• Add HashValidator::validate_for_contract: accepts only 64-char SHA-256 hex, explicitly rejects 128-char SHA-512 with UnsupportedAlgorithm, normalizes to lowercase
• Add HashValidator::hex_to_bytes32: converts a validated hex string to [u8; 32] for BytesN<32> contract calls
• Add CacheKey::verification() normalizing constructor: lowercases hash on construction so mixed-case inputs always resolve to the same cache entry
• Update StellarClient::verification_cache_key to use the normalizing constructor
• Update README canonical hash encoding section with API usage examples

Testing

• cargo test passes locally
• cargo build --target wasm32-unknown-unknown --release succeeds
• New tests added for new functionality (if applicable)

Checklist

• My code follows the project's coding conventions
• I have performed a self-review of my code
• I have commented my code where necessary
• No unused imports or dead code remains
• CI checks pass (build + test + wasm)