refactor: replace hardcoded dust limits with bdk-wasm's is_dust()

[darioAnongba]

Summary

The dust validation in packages/snap/src/handlers/validation.ts currently hardcodes dust thresholds per address type:

function getDustLimitSats(addressType: AddressType): bigint {
  switch (addressType) {
    case 'p2wpkh': return 294n;
    case 'p2pkh':  return 546n;
    case 'p2sh':   return 540n;
    case 'p2wsh':  return 330n;
    case 'p2tr':   return 330n;
    default:       return 546n;
  }
}

With bitcoindevkit/bdk-wasm v0.3.0, the following dust check methods are now available:

• Amount.is_dust(script) — check if an amount is below the dust limit for a given script
• ScriptBuf.minimal_non_dust() — get the minimum non-dust amount for a script
• ScriptBuf.minimal_non_dust_custom(fee_rate) — same with a custom dust relay fee rate

These were added in bitcoindevkit/bdk-wasm#13 and delegate to BDK / Bitcoin Core's actual dust relay policy logic.

Suggested change

// Before: hardcoded switch + manual comparison
const sats = Amount.from_btc(Number(amountInBtc)).to_sat();
const min = getDustLimitSats(account.addressType);
if (sats < min) { ... }

// After: delegate to BDK
const amount = Amount.from_btc(Number(amountInBtc));
if (amount.is_dust(account.publicAddress.script_pubkey)) { ... }

Benefits

• Correct by construction — matches Bitcoin Core's exact dust relay policy
• Future-proof — automatically handles new script types without code changes
• Less maintenance — no hardcoded constants to keep in sync
• Simpler — removes ~25 lines of code

Dependency

Requires upgrading @metamask/bitcoindevkit to a version built from bitcoindevkit/bdk-wasm v0.3.0+.

Existing dust limit tests in RpcHandler.test.ts should continue to pass since BDK computes the same thresholds (e.g. 294 sats for p2wpkh).