Add backend validation for transaction hash parameters

[mimijuwonlo-commits]

Area

Backend API security

Problem

Transaction hash lookup endpoints should reject malformed identifiers early. This avoids noisy logs, unnecessary database work, and ambiguous client errors.

Acceptance Criteria

• Validate hash length and character set before lookup
• Return stable validation errors for malformed input
• Add tests for empty, oversized, invalid, and valid hashes
• Update OpenAPI parameter schema

Validation

Run backend transaction route tests and OpenAPI drift check.