Generate and propagate simulation telemetry envelopes for Modal simulation requests

[anth-volk]

This issue tracks the first split of the Grafana observability rollout in policyengine-api.

Scope:

• generate a _telemetry envelope when submitting simulation jobs to the Modal simulation API
• include run_id, process_id, requested timestamp, config hash, geography classification, and traceparent when available
• preserve the existing _metadata payload while adding _telemetry
• propagate run_id through the Modal client response object and submission logs
• add targeted fixtures and unit coverage for the new telemetry payload behavior

This PR should stay intentionally narrow: no OTLP export, no stage metrics, and no new internal observability API yet.