feat: migrate to caip-25 multichain api when production-ready #12123
Description
When MetaMask's CAIP-25 Multichain API (wallet_createSession / wallet_invokeMethod) graduates from Flask to production, consider migrating from per-chain Wallet Standard integrations to the unified Multichain API.
Benefits: single session for all chains, unified method invocation, event-based account/session changes, cleaner code. Currently Flask-only.
Depends on MetaMask shipping CAIP-25 in production extension.