Add x402 HTTP payment protocol: client, spec, and registry integration

[tboutron]

Motivation

• Introduce support for an HTTP-native payment challenge/settlement protocol (x402) to allow services to respond with 402 challenges and enable automatic settlement and retry semantics.

Description

• Add an integration spec for payments.x402 with configuration and secret schema in src/integrations/providers/x402.ts and register it in the default provider registry via registerX402Integration.
• Implement the X402PaymentsClient (src/impls/x402-payments.ts) and shared parsing/utility code (src/impls/x402-payments.shared.ts) to parse challenge headers, create payment intents via an underlying PaymentsProvider, and attach proof tokens to retried requests.
• Extend the payments contract types with X402PaymentRequirement, X402PaymentProof, X402FetchRequest, and X402HttpPaymentClient in payments.ts to expose the new client API surface.
• Wire the implementation into the provider implementations and exports by adding ./impls/x402-payments to the package exports and export * from './x402-payments' in the impls index, and add a createX402PaymentsClient helper to provider-factory.ts for constructing the client using an existing settlement PaymentsProvider.
• Add unit tests for the client logic in x402-payments.test.ts and add a registration test to providers.test.ts; update package exports entries for the newly added modules.

Testing

• Ran the new unit tests for the x402 client via bun:test targeting src/impls/x402-payments.test.ts, and the tests passed.
• Ran the updated providers.test.ts that checks registration of payments.x402 via bun:test, and the test passed.

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
contractspec-library-api	Ready	Preview, Comment	Mar 26, 2026 9:01pm
contractspec-library-landing	Error		Mar 26, 2026 9:01pm

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• Commit unit tests in branch codex/add-x402-protocol-integration

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📋 ContractSpec Impact Analysis

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