Add observability scaffolding and telemetry propagation for the simulation gateway

[anth-volk]

This issue tracks the first split of the Grafana observability rollout in policyengine-api-v2.

Scope:

• add the shared observability scaffolding and contracts in policyengine-fastapi
• expose the new observability settings in the simulation API project without changing runtime behavior
• accept and normalize the internal _telemetry envelope in the Modal gateway models
• pass the telemetry envelope through the gateway to the worker
• strip _telemetry before SimulationOptions validation in the worker while preserving correlated logging
• add targeted tests for the new contracts, settings, gateway models, and telemetry passthrough

This PR should stay intentionally narrow: no OTLP export, no Grafana wiring, and no stage-timing instrumentation yet.