fix(blockrun): remove default x402 payment cap

[jjcc123312]

Problem / background

BlockRun x402 requests are rejected before signing whenever the upstream payment quote exceeds the hard-coded 5 USDC default cap. Request 202608110702314015628818268d9d62ioY87Dv quoted 13.8367 USDC for /v1/responses, causing the gateway to return HTTP 500 with amount 13836700 exceeds per-call cap 5000000.

Evidence / root cause

SignX402Payment and the image branch of Adaptor.DoRequest passed maxAmountAtomicUSDC into the shared payment validator. assertAmountWithinCap rejected the upstream 402 amount before creating an ERC-3009 signature, so the request failed without an on-chain charge.

Scope / design

• Remove the default amount ceiling for BlockRun Chat, Responses, and image requests by passing a nil amount cap.
• Keep positive decimal amount validation.
• Preserve the Base/Base Sepolia network allowlist, Base USDC asset check, pay-to address validation, and authorization-window limits.
• Preserve explicit caller-provided caps. The Seedance video path therefore retains its existing 10 USDC limit.
• Add a regression test using the observed 13.8367 USDC quote and retain explicit-cap coverage.

Impact and risks

This intentionally expands wallet exposure: the default BlockRun paths will sign any positive amount advertised by an otherwise valid upstream 402 response. A compromised BlockRun service or TLS boundary could request a very large payment. The network, asset, recipient-shape, time-window, and ERC-3009 nonce protections remain, but they do not bound the single-transfer amount.

The change is multi-node safe: it introduces no shared mutable or process-local coordination state. Every router instance applies the same stateless validation after deployment, and ERC-3009 nonces continue to make each authorization single-use.

Validation

• go test ./relay/channel/blockrun -run "Test(ValidatePaymentOption|LooksLikeEthAddress|ExtractPaymentRequired|ParsePrivateKey|CacheRequestBody|X402)" -count=1
• go test ./relay/channel/blockrun -run "^$"
• go test ./relay/channel/task/blockrunseedance/...
• git diff --check
• GitNexus change detection: one expected affected process, DoRequest -> SetupLogger; medium risk due relay-path behavior change.

The full go test ./relay/channel/blockrun/... suite still has existing image polling/download failures because the test environment leaves the global HTTP client uninitialized. The x402-specific tests and package compilation pass.

Acceptance criteria

• A valid BlockRun 402 quote of 13.8367 USDC is accepted on Chat/Responses/image paths.
• Invalid, zero, and negative amounts remain rejected.
• Explicit amount caps remain enforceable for callers such as Seedance video.
• Network, asset, pay-to, and authorization-window guards remain unchanged.

Production deployment recommendation

• Router deploy: required.
• Reason: this changes BlockRun relay payment signing for /v1 model and image traffic.
• Other deploy targets: newapi-console, newapi-web, Terraform, and Cloudflare are not involved.
• Risk / validation: high financial exposure if upstream advertises an abnormal amount; smoke-test a paid BlockRun Responses request in staging and confirm the quote, signature, response, and settlement log before production rollout.