fix(rpc): align SSZ witness response decoding with current draft

[JackCC703]

🗒️ Description

This updates the experimental REST+SSZ witness response decoder on projects/zkevm from the older flat NewPayloadWithWitnessResponse shape toward the newer PayloadStatusWithWitness draft shape, while keeping the existing simulator flow unchanged.

🔗 Related Issues or PRs

#2724 and MariusVanDerWijden/execution-apis#1

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  just static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered updating the online docs in the ./docs/ directory.

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