Set Default Gateway Wallet

Set the default wallet for a chain in Gateway.

When multiple wallets are configured for a chain, this endpoint allows switching which wallet is used as the default for operations.

Args: request: Contains chain and wallet address to set as default

Returns: Dict with success status and updated wallet info.

Example: POST /accounts/gateway/wallet/set-default { "chain": "solana", "address": "82SggYRE2Vo4jN4a2pk3aQ4SET4ctafZJGbowmCqyHx5" }