fix: reject non-USDC x402 payment challenges

[scosemicolon]

Summary

• reject x402 SVM payment challenges whose asset is not the Solana USDC mint
• preserve the new asset_mismatch trust-policy reason through the wrapped @x402/fetch error message
• add a regression test for non-USDC challenge rejection

Verification

• git diff --check -- src/http.ts src/errors.ts tests/trust-policy.test.ts

I did not run the full Vitest suite locally because this shallow inspection clone has no installed dependencies, and I did not install packages.