Align simulation API with Populace bundle metadata

[anth-volk]

Fixes #589
Fixes #587

Summary

This PR moves the simulation API toward the Populace-based .py bundle contract:

• Uses bundled Populace datasets in simulation integration payloads instead of legacy policyengine-us-data / policyengine-uk-data dataset aliases.
• Reports Populace bundle metadata from gateway responses using the certified data release version where appropriate.
• Removes legacy dataset aliases from the simulation API release bundle metadata.
• Removes runtime tests that assumed split US state/district H5 dataset files.
• Keeps custom dataset values covered as request metadata without using legacy datasets in computation/integration tests.
• Updates the simulation API project to .py 4.18.7, which certifies policyengine-us==1.729.0, policyengine-uk==2.89.2, and policyengine-core==3.28.0.

Bundle Values

The local bundle extractor now resolves:

policyengine_version=4.18.7
policyengine_core_version=3.28.0
us_version=1.729.0
us_data_version=populace-us-2024-f0af251-703bd81a565c-20260620T201958Z
uk_version=2.89.2
uk_data_version=populace-uk-2023-dd68c73-4aa4b14-20260619T023711Z

Validation

uv run pytest tests/test_policyengine_dependency_source.py tests/test_modal_scripts.py tests/test_update_version_registry.py tests/test_release_bundle.py tests/test_dataset_uri.py tests/test_hf_dataset.py -q
93 passed

uv run pytest tests/gateway -q
131 passed, 1 warning