[node-sdk] Expand integration test coverage

[KayleeWilliams]

Why

packages/node-sdk is 2,847 src LOC vs 495 test LOC (~17%). This is the SDK customers actually import - an untested public API erodes trust on first bug and makes every refactor risky.

Scope

• One contract test per endpoint (intake, init, status, requests, verification, subjects, appeals, artifact delivery, notifications, webhooks, audit, fulfillment, extensions, policies)
• Error envelope shape tests for each error code
• Timeout behavior (slow endpoint, no response, network error)
• Retry behavior (if the SDK retries - document and test; if not, document that)
• Malformed response handling (non-JSON, truncated, unexpected shape)
• 4xx and 5xx paths return typed errors
• Target: ~40% test LOC ratio

Priority: P1 - beta blocker

[coderabbitai]

🔗 Related PRs

#7 - feat(sdk,cli): achieve 100% endpoint coverage across CLI and Node SDK [merged]

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