feat(sdk-coin-canton): add comprehensive payload validation

- Replace unsafe fallback paths in Transaction.signablePayload
- Replace unsafe fallback paths in WalletInitTransaction.signablePayload
- Add validation for all required IMS response fields
- Add base64 encoding validation with detailed error messages
- Add unit tests covering all error conditions (12 total tests)
- Remove duplicate test cases that were causing CI failures
- Fix TypeScript type errors in test objects

Ticket: WP-8187

Author: derranW26