examples: add HOLD coverage for payment guard service

[safal207]

Motivation

• Ensure the Agent Payment Guard HTTP service example explicitly covers HOLD behavior for recurring payments that lack approval so mode semantics are fully demonstrated.
• Provide explicit assertions for both enforce and shadow modes and make the service-level audit expectations deterministic.

Description

• Update examples/agent-payment-guard/run_service_check.sh to POST the payment_proposal.recurring_without_approval.json fixture and assert HOLD + RECURRING_PAYMENT_REQUIRES_APPROVAL in enforce mode.
• Add a matching POST and assertions for HOLD in shadow mode, verifying execution_allowed differs by mode while would_block remains true.
• Update the service audit assertion to expect count==4 and verify the explicit decision sequence ACCEPT, BLOCK, HOLD, HOLD.
• Preserve hash-chained audit verification by keeping the python3 scripts/verify_audit_log.py .proofpath/audit.jsonl check unchanged.

Testing

• Ran bash examples/agent-payment-guard/run_demo_check.sh; it passed.
• Ran bash examples/agent-payment-guard/run_service_check.sh; it passed and exercised both HOLD cases.
• Ran python3 scripts/verify_audit_log.py .proofpath/audit.jsonl; hash-chain verification passed.

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Codex Task