Use .py stack metadata for simulation release metadata

[anth-volk]

Goal

Update the policyengine-api-simulation project to understand the new .py stack metadata while keeping a fallback for existing country release manifests during the transition.

Work

• Prefer policyengine.stack metadata when reporting the .py stack version and model package versions.
• Keep existing release-manifest fallback behavior for environments that do not yet have stack metadata available.
• Avoid adding any dependency on policyengine-bundles for runtime API metadata.
• Add tests for both paths: stack metadata available and fallback manifest only.
• Confirm dataset artifact revisions are not accidentally changed when stack package pins differ from data artifact versions.

Acceptance criteria

• API simulation release metadata reports the official .py stack version when available.
• Existing release-manifest behavior remains compatible during rollout.
• No API deployment step is required just to publish a new stack, unless the API intentionally chooses to consume a newer .py release.