Add contract data validation before submission

[Just-Bamford]

Problem

Contract data stored without validation. Invalid data may be stored, causing contract failures.

Solution

Implement \�alidateContractData(contractId, key, value, type?)\ in \src/soroban/index.ts. Validates data format and type before storage.

Acceptance Criteria

• Function added to \src/soroban/index.ts\
• Validates data format per type
• Returns validation result
• Tests cover valid/invalid data

Note for contributors

Reference contract data types. Add tests to \src/tests/soroban.test.ts.