fix(payment-requests): validate memo length against Stellar 28-byte limit (#49)

[Simultech369]

Summary

Resolves #49 by enforcing Stellar's on-chain 28-byte MEMO_TEXT limit (stellar_xdr::curr::Memo::Text / StringM<28>) on payment_requests.memo at both the application service layer and database layer.

Changes

1. Service Validation (src/services/payment_request.rs):
   • Added canonical constant pub const MAX_MEMO_TEXT_BYTES: usize = 28;.
   • Added PaymentRequestService::validate_memo(memo: Option<&str>) -> AppResult<()>.
   • Enforced validation in PaymentRequestService::create before preparing or executing the database query.
   • Measures UTF-8 byte length via memo.as_bytes().len(), guarding against multi-byte character overflow where character count <= 28 but byte length > 28.
2. Database Migration (migrations/012_payment_requests_memo_length_check.sql):
   • Added defense-in-depth constraint:

     ALTER TABLE payment_requests
     ADD CONSTRAINT check_payment_requests_memo_length
     CHECK (memo IS NULL OR octet_length(memo) <= 28);

   • Uses octet_length for byte-exact enforcement in PostgreSQL while preserving nullability for optional memos.
3. Unit Tests (src/services/payment_request.rs):
   • memo_at_exact_28_ascii_bytes_is_accepted (28 bytes accepted).
   • memo_exceeding_28_ascii_bytes_is_rejected (29 bytes rejected with AppError::Validation).
   • memo_multi_byte_utf8_exceeding_28_bytes_is_rejected (10 3-byte unicode symbols = 30 bytes, 10 chars, rejected with AppError::Validation).
   • memo_multi_byte_utf8_within_28_bytes_is_accepted (9 3-byte unicode symbols + 1 ASCII byte = 28 bytes total, accepted).
   • memo_none_and_empty_is_accepted (None and "" accepted).

Closes #49

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Payout Address (Stellar USDC): GCK7WCZCOFNYXTH74KADMAIREIJKHIAX4ISJSGXZ4IF2MIRGAFLT4BTH
EVM (Base USDC): 0xEa3A353Fb3fc88DB8F0FA1E6Ea541FEdc9F5F0E7

[Simultech369]

Hi @abayomicornelius — PR #81 is fully tested and ready for review. It resolves #49 by enforcing Stellar's 28-byte UTF-8 MEMO_TEXT limit at both the service layer (validate_memo) and database schema level via migration 012 (CHECK (memo IS NULL OR octet_length(memo) <= 28)), with all 5 boundary unit tests passing cleanly in cargo test.

Mergeable and ready for review whenever convenient!

• Stellar Payout: GCK7WCZCOFNYXTH74KADMAIREIJKHIAX4ISJSGXZ4IF2MIRGAFLT4BTH
• Base USDC: 0xEa3A353Fb3fc88DB8F0FA1E6Ea541FEdc9F5F0E7