fix: prevent replay of fulfilled mpp payments

[dvansari65]

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[dvansari65]

• add replay protection for fulfilled MPP payments in the server middleware
• reject duplicate payment proofs with a non-retryable verification_failed 402 response
• mark proofs as consumed only after the protected upstream response succeeds
• wire a default replay store into pay server start so the protection is active out of the box

[lgalabru]

hey @dvansari65! thanks for opening this PR.
This issue was caught and fixed at the mpp-sdk level a few days ago.
PR #362 is pulling the latest version of this build which is fixing this issue.
Thanks for looking into this, feel free to re-open if you're seeing something else.

[dvansari65]

@lgalabru glad to hear this, while doing this , I thought I am doing wrong cuz I was reading about this 402 payment security check on on some avalanche official page so I thought , I can do something in solana/pay repo.