feat(agent-fetch): route L402 payments through zbd.ai shield proxy

[andrerfneves]

Summary

• route L402 invoice payments through /api/shield/l402 before direct ZBD payment execution
• add deterministic handling for shield 403 allowance_exceeded and 202 pending_approval responses
• add graceful network-only degradation to direct payment path and test coverage for shield/fallback flows

Verification

• npm --prefix agent-fetch run build
• npm --prefix agent-fetch test

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[github-actions]

🎉 This PR is included in version 1.1.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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