feat: replace manual Solana x402 with official x402 SDK

[notorious-d-e-v]

Summary

• Replace ~200 lines of manual Solana transaction construction (ATA derivation, signing, instruction building) with the official x402 Python SDK (pip install x402[svm])
• This fixes both bugs from fix: correct Associated Token Program ID for Solana payments #2 (wrong ATA program ID + missing v0 signing prefix) by delegating to the battle-tested SDK maintained by Coinbase
• Net reduction of ~210 lines of code while gaining future protocol compatibility

Changes

File	Change
pyproject.toml	Replace solders+base58 with x402[svm] in solana optional deps
blockrun_llm/x402.py	Remove entire Solana section (~180 lines), keep EVM code unchanged
blockrun_llm/solana_client.py	Use x402ClientSync + KeypairSigner + ExactSvmScheme for payment signing
tests/unit/test_x402.py	Replace manual Solana payload tests with x402 SDK integration tests

How it works

# Before: ~150 lines of manual transaction building
payload = create_solana_payment_payload(
    private_key=key, recipient=addr, amount=amt, fee_payer=fp, ...
)

# After: x402 SDK handles everything
payment_required = decode_payment_required_header(header)
payment_payload = x402_client.create_payment_payload(payment_required)
encoded = encode_payment_signature_header(payment_payload)

Notes

• The solana optional dependency now requires Python ≥3.10 (x402 SDK requirement). Base EVM functionality still works on Python 3.9.
• The x402 SDK adds a memo instruction to transactions (optional per protocol spec, accepted by facilitator)
• Backwards-compatible key handling: supports both full 64-byte keypairs and 32-byte seeds via fallback

Test plan

• All 89 unit tests pass
• Live Solana payment verified — tx on Solscan

Closes #3