feat: cap message field at 2000 chars with validation and tests (Closes #1504)

[waterWang]

Summary

Adds server-side validation for the optional message field used in remittance and emergency-transfer flows. The message is capped at 2000 characters to prevent abuse and ensure database consistency.

Changes

• utils/validation.ts: Added validateMessage() function that checks character length (not byte length, since this is a user-facing message, not a Stellar memo)
• app/api/remittance/build/route.ts: Wire message validation into the remittance build endpoint
• app/api/v1/remittance/emergency/build/route.ts: Wire message validation into the emergency transfer build endpoint
• types/emergency-transfer.ts: Added INVALID_MESSAGE error code and optional message field to EmergencyTransferRequest
• tests/unit/remittance/build-validation.test.ts: Added comprehensive unit tests for validateMessage() covering:
  • Undefined/empty string (returns null — message is optional)
  • Message within 2000 chars (accepts)
  • Message exceeding 2000 chars (rejects with structured error)
  • Message exactly 2000 chars (boundary case)
  • Multi-byte characters (UTF-8 safety)
  • Emoji characters

Implementation Details

• Rejects bad input at the API boundary (not deep in the call graph)
• Errors returned as structured typed errors (EmergencyTransferErrorCode.INVALID_MESSAGE), never stringly-typed 400
• Follows the same pattern as existing validateMemo() and validateAmount() functions

Closes #1504