feat(canton): implement getSignablePayload override for EdDSA extended payload

[derranW26]

Summary

Implements the upstream fix for Canton EdDSA signature verification by overriding getSignablePayload() in the Canton coin class to construct the proper extended payload format.

Problem

Canton's HSM must sign a structured payload containing transaction metadata concatenated with the signable hash, not just the hash alone. The base implementation was returning only the hash, causing signing/verification mismatches.

Solution

Override getSignablePayload() in Canton coin to:

1. Parse the serialized transaction (base64 JSON)
2. Extract the signable hash (preparedTransactionHash)
3. Build extended payload based on transaction type:
   • WalletInitBuilder: [txnType (optional)] || itemCount || [lenOfTx || tx]... || signableHex
   • TransactionBuilder: itemCount || lenOfTx || preparedTransaction || signableHex

Impact

• Eliminates duplicate payload construction logic across microservices
• Provides single source of truth in BitGoJS for Canton extended payload format
• Allows signing code to automatically use correct bytes
• Enables microservices code simplification (remove 80+ lines)

Test Plan

• TypeScript compilation succeeds
• Unit tests for Canton coin
• Integration tests for signing/verification flows