Compatibility with another mpp #24
Description
Motivation:
curl --request POST \
--url https://mpp.quicknode.com/solana-mainnet \
--header 'Content-Type: application/json' \
--data '{"jsonrpc":"2.0","id":1,"method":"getSignaturesForAddress","params":["CzzWWaQtz8oCcEPiRi1ziz9djWqYf7WgZK9rtbJSMEef",{"limit":10}]}'
then the response header was containing eyJhbW91bnQiOiIwLjAwMSIsImN1cnJlbmN5IjoiRVBqRldkZDVBdWZxU1NxZU0ycU4xeHp5YmFwQzhHNHdFR0drWnd5VER0MXYiLCJtZXRob2REZXRhaWxzIjp7ImRlY2ltYWxzIjo2LCJtaW50IjoiRVBqRldkZDVBdWZxU1NxZU0ycU4xeHp5YmFwQzhHNHdFR0drWnd5VER0MXYiLCJuZXR3b3JrIjoibWFpbm5ldC1iZXRhIiwicmVjaXBpZW50IjoiNmJNWkRHYVdMb0pFVkN3UzZSQ05hcWZTM1VpcHFHMmQ2bVVORXFwNktRWjUiLCJyZWZlcmVuY2UiOiJCdVlHZkVhcXFHZUJXYzVldzJFZjZYRWJSaG9mNzF6Z1lZOGpGdlUxNWNxcyJ9fQ ->
{"amount":"0.001","currency":"EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v","methodDetails":{"decimals":6,"mint":"EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v","network":"mainnet-beta","recipient":"6bMZDGaWLoJEVCwS6RCNaqfS3UipqG2d6mUNEqp6KQZ5","reference":"BuYGfEaqqGeBWc5ew2Ef6XEbRhof71zgYY8jFvU15cqs"}}
Normally it would have contain "amount":"1000" because USDC token has 6 decimals. The entire problem is because quicknode is using another solana mpp implementation (https://www.quicknode.com/docs/build-with-ai/mpp-payments), but independently of it, I want for my agent to be able to pay for all by simply using official mpp, without worrying about support for another mpp.
The main problem was that the @solana/mpp client called BigInt(amount) directly on the challenge amount string. When a server sends a human-readable decimal like "0.001" instead of atomic units like "1000", BigInt("0.001") throws a SyntaxError, making the payment fail entirely.