fix(demos): abort the roboledger load when an event or agent write fails

[jfrench9]

Summary

The second copy of the defect #1318 fixed in the scenario runner. just demo-roboledger loads Cascade Advisory through examples/roboledger_demo/main.py, which carries its own duplicate of the event and agent loops — and both swallowed a failed write into a counter and carried on.

The consequences are the ones #1318 spelled out, and the CI run that started all this demonstrated them: bill_payment posts two linked events, so a failure between them strands an AP balance with no discharge (bill_received: 95 against bill_paid: 94 in that log); a dropped invoice leaves its later payment with no obligation to discharge; and a dropped agent is quieter still, because agent_lookup.get() then returns None and every event for that counterparty posts with no agent_id — the REA linkage the demo exists to show, silently absent. The run would then close periods and materialize over the gap and still exit 0.

Locally this can never fire: RATE_LIMIT_ENABLED=false, so nothing is rejected. But this demo also runs against a deployed API via DEMO_API_URL, which is exactly the configuration where the showcase episode lost 25 of 383 events while reporting success.

Changes

examples/roboledger_demo/main.py only.

• create_agents — raises instead of counting, naming how many agents were created first.
• create_business_events — raises instead of counting, naming how many events were loaded first.
• The genuine data skip (a transaction whose lines resolve to no CoA element) stays non-fatal and is now counted as unmapped, reported separately. Folding it together with API failures under "Skipped N events (missing elements or errors)" is what made the original invisible.

Both comments point at _scenario/runner.py so the two copies stay legible as the pair they are.

Testing

• just test-code — ruff, format, basedpyright clean.
• just demo-roboledger run end-to-end against the local stack: exit 0, no warnings, 17 agents, 317 events / 683 line items, with bill_received 85 = bill_paid 85 and invoice_issued 47 = payment_received 47 — the balanced pairs that were asymmetric in the failing CI log.
• The abort path itself is not exercised: nothing is rejected locally with rate limiting off. It is the same code shape already reviewed in fix(demos): abort the scenario load on a failed write, and pull through client 1.13.1 #1318.