feat: add optional core routing decision gate

[mrnicholasbcarter-code]

Summary

• add optional Verdict core decision endpoint configuration to LlmGateNode
• validate canonical routing decisions before adapting them to middleware metadata
• fail closed with 503 when configured core routing is unavailable, malformed, or denied
• preserve heuristic fail-open routing when the endpoint is not configured

Verification

• npm test -- --runInBand
• npm run build
• npm run format:check
• npm run lint
• npm run verify:package

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