Add batch settlement beta facilitator image

[bussyjd]

Summary

Adds EIP-155 batch-settlement facilitator support to the Obol x402-rs fork and introduces a beta image track for the Prometheus overlay facilitator image.

Changes

• Cherry-picks the batch-settlement facilitator implementation.
• Publishes beta overlay builds from beta/** branches.
• Adds mutable beta image tags:
  • ghcr.io/obolnetwork/x402-facilitator-prometheus-overlay:beta
  • ghcr.io/obolnetwork/x402-facilitator-prometheus-overlay:beta-<workspace-version>
• Keeps existing main, release tag, latest, next, and SHA tagging behavior unchanged.

Testing

• cargo test -p x402-chain-eip155 --features facilitator
• cargo test -p x402-facilitator-prometheus-overlay
• cargo fmt --check
• git diff --check

Deployment

The beta branch push triggered the Prometheus overlay image workflow. The matching infrastructure change points the x402 facilitator Helm chart at the beta image tag and enables the v2-eip155-batch-settlement scheme.