Add unit tests for web x402 client request and receipt helpers

[AnkanMisra]

Summary

The web x402 client module owns the browser-side HTTP request shape, payment-challenge parsing, signed retry headers, and receipt-header decoding, but it has no direct unit tests. These helpers are used by the paid summarize flow and can be tested without a wallet or running service.

Current behavior

The module currently:

• builds the gateway summarize request;
• reads the paymentContext from a 402 response;
• converts a payment signature into the three X-402-* retry headers;
• reads successful summary responses;
• drops missing or malformed receipt headers without discarding the successful summary.

Those contracts are only exercised indirectly by higher-level code.

Desired behavior

Add deterministic Bun tests that lock down the existing request, challenge, header, and response behavior. Production behavior should remain unchanged.

Key interfaces

• getGatewayUrl() — configured URL and localhost fallback.
• postSummarize() — request URL, method, JSON body, content type, and caller-supplied headers.
• readPaymentChallenge() — valid challenge extraction and missing-context rejection.
• buildSignedHeaders() — exact signature, nonce, and numeric timestamp header values.
• readSummarizeSuccess() and safeDecodeReceiptHeader() — valid, absent, malformed, and wrong-shape receipt headers.

Acceptance criteria

• Tests verify the configured gateway URL and the localhost fallback.
• Tests verify the exact summarize request method, URL, JSON body, content type, and custom-header preservation using a mocked fetch.
• Tests verify valid payment-context parsing and rejection when paymentContext is missing.
• Tests verify the exact three signed retry headers, including timestamp string conversion.
• Tests verify successful responses with a valid receipt header and with no receipt header.
• Malformed base64, invalid JSON, incomplete receipt objects, and wrong field types return a null receipt without losing the summary.
• Tests do not require a live gateway, verifier, wallet, Redis, or AI provider.
• cd web && bun run test:unit && bun run typecheck passes.

Out of scope

• Changing the x402 wire contract or EIP-712 fields.
• Testing wallet prompts or React hook state transitions.
• Refactoring the SDK implementation.
• Changing how malformed receipts are reported to users.

Blocked by

None — can start immediately.

[Arjun8242]

I want to work on it.
could you assign it to me?