Replace manual x402 Solana implementation with official x402 Python SDK

[notorious-d-e-v]

Problem

blockrun_llm/x402.py manually reimplements the x402 client-side payment logic for Solana (ATA derivation, transaction building, v0 signing, blockhash fetching). This has led to two bugs that break all Solana payments:

1. Wrong Associated Token Program ID — ATokenGPvbdGVxr1b2hvZbsiqW5xWH25efTNsLJe1bRS instead of ATokenGPvbdGVxr1b2hvZbsiqW5xWH25efTNsLJA8knL (causes recipient_mismatch)
2. Missing v0 version prefix when signing — bytes(message) instead of b'\x80' + bytes(message) (causes SignatureFailure)

Both are fixed in #2, but the root cause is maintaining a parallel implementation of logic that already exists in the official SDK.

Proposed Solution

Replace the manual Solana x402 implementation with the official x402 Python SDK (v2.3.0+):

pip install "x402[svm]"

The official SDK's ExactSvmScheme already handles:

• Correct ATA derivation (including Token-2022 support)
• v0 message signing with 0x80 prefix
• Token program detection from on-chain mint owner
• Decimals fetched from mint data (not hardcoded)
• Memo instruction with random nonce for uniqueness
• Compute budget instructions

Example integration

from x402 import x402ClientSync
from x402.mechanisms.svm.exact import ExactSvmScheme
from x402.mechanisms.svm.signers import KeypairSigner

signer = KeypairSigner.from_base58(private_key)
client = x402ClientSync()
client.register("solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp", ExactSvmScheme(signer=signer))

# Create payment payload from 402 response
payload = client.create_payment_payload(payment_required)

This would replace ~200 lines of manual transaction construction in x402.py and eliminate an entire class of bugs by staying in sync with the facilitator's expectations.

Benefits

• Correctness: maintained alongside the x402 facilitator, so client and server always agree on transaction format
• Future-proof: new features (Token-2022, new instructions) are handled upstream
• Less code to maintain: removes _get_ata(), _get_latest_blockhash(), create_solana_payment_payload(), and all the Solana constants