feat(x402): optional x402 payment middleware for HTTP/SSE transports

[wtfsayo]

Problem

• Pay-protected MCP tools require payment headers; current gateway HTTP/SSE transports cannot attach x402 proof.

Solution

• Introduce optional x402 middleware, enabled per-server via x402Middleware (boolean | object). When enabled, HTTP/SSE transports use a lazy-created x402-aware fetch to inject payment headers.

Key changes

• src/types.ts: Add x402Middleware to McpServerConfig with wallet config and maxValueMicroUSDC.
• src/transport-factory.ts: Wrap HTTP/SSE transports with x402 fetch when enabled; propagate headers/apiKey; strong typed stdio options.
• src/aggregator-server.ts & src/server-manager.ts: Log x402 enablement and per-server limits.
• src/x402.ts: Add helper to create x402 fetch and feature flag checks.
• package.json: Add x402/x402-fetch deps; x402 test scripts.
• examples/: Add Context7 x402 production, x402 example; update mixed-transports.
• tests/: Add unit and e2e tests/configs; shell harness.
• tsconfig.json: Switch to NodeNext for ESM compatibility.
• env.example: Document wallet env var names.

Notes

• Backwards compatible: x402 is disabled by default unless x402Middleware is set/enabled.
• README.md changes deferred; bun.lock excluded.

Testing

• npm run test:x402 and npm run test:x402:unit cover core flows; integration configs under tests/configs/*.

Follow-ups

• Document x402 configuration in README.md and add CI job for x402 tests.

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