Fix x402 MCP payment retry metadata

[mohamedkuch]

Summary

• decode the signed x402 payload before retrying a payment-required MCP tool result
• attach the payload to params._meta["x402/payment"] for regular, task, and detached calls
• preserve caller metadata and clear the cached signature after the bounded retry
• add regression coverage for payment payload decoding

Live compatibility proof

Tested the patched build against https://api.utilia.ink/base/mcp with released mcpc 0.5.1 as the baseline. The initial call returned payment-required terms, the patched retry settled exactly $0.002 USDC on Base, and the tool returned a successful result.

Transaction: 0x1815f8271c194700aa3d178ca1ff4bc76d4c0e82bdea4e0e369008cbfd3e63cb

This was a bounded operator compatibility test. It was not organic traffic.

Validation

• pnpm run build
• pnpm run lint with 5 pre-existing warnings and 0 errors
• pnpm run test:unit: 867 passed
• Node E2E: 47 passed
• Bun E2E: skipped because Bun is not installed in the test environment
• real Base x402 MCP call: passed with one settlement

Fixes #343

[mohamedkuch]

@jancurn, could you please review PR #344 when convenient? The change is limited to propagating the freshly decoded x402 payload through params._meta["x402/payment"] on the immediate paid retry while preserving caller metadata. It adds focused regression tests, and the full Node unit and E2E suites, build, formatting, lint, and PR-diff secret scan pass. It does not introduce or broaden session-level payment-signature reuse.