Split the entrypoint app and its deployment asset tests under the file-length limit

changelog_entry.yaml

@@ -16,6 +16,7 @@
       - Ran Stage 12 US national simulations as 20 region-group calls each, and built an independent, immutable Stage 12 cache of three annual US datasets and 60 current-law baselines before each deployment, publishing it only after a recorded determinism check and recording its release in the v2 version manifest under a new key, while the existing key keeps a copy that earlier readers accept
       - Made the budget refuse a household count outside its country's plausible range (above 200 million in the US or 60 million in the UK, negative or not finite), raising ImplausibleEntityCountError instead of publishing a count like #767's
     changed:
+      - Moved the entrypoint's backend calls, response telemetry and comparison dispatch out of app.py (925 lines, now 583) into entry_runtime.py, which app.py re-exports and which still logs under the policyengine_simulation_entry.app logger; the routes stay in app.py, so FastAPI and ruff read their annotations as before; the 2,251-line deployment asset test file is split into seven test files and a support module named test_deployment_assets_support.py, the rules registry names each moved test by its new file, and its guard-test discovery now covers tests/test_deployment_assets*.py
       - Moved the worker's credential setup, request year and policy parsing, and region resolution out of simulation_runtime.py (1,111 lines, now 651) into gcp_credentials.py, request_policy.py and region_resolution.py; simulation_runtime.py keeps dataset selection and loading, simulation building, local-area breakdowns and the run itself, which tests replace by name there, and re-exports the moved names, so every caller still imports them from simulation_runtime; the shared helpers _parse_year, _normalise_policy, _compile_interval_policy, _runtime_policy (now wrapper_policy), _resolve_region, _resolve_dataset_reference and _require_local_area_breakdown are now public, and the credential messages still log under the simulation_runtime logger; the 3,048-line output builder test file is split into seven test files and a module of shared fakes, all named test_simulation_output_*.py so the weighted-frame rule still checks them
       - Split the artifact precompute (precompute.py, 931 lines) and the Stage 12 cache precompute (stage12_cache/precompute.py, 844 lines) each into planning, dataset-building and baseline modules, with the manifest and run_precompute left in the original module, which re-exports every public name, so the Modal precompute apps and their tests import from the same paths; the cache precompute's _scoped_planned_simulation and _baseline_identity are now public, and the worker validation imports scoped_planned_simulation from its new module; the 1,835-line precompute test file is split into four, and the cache precompute tests patch each name in the module whose code reads it
       - Split the 4,709-line Stage 12 runtime test file into thirteen test files by behaviour (dataset loading, provenance, the single worker and its failures, aggregates, the coordinator and its failures, runtime store retries, failure records, cancellation, child state errors and the wait window) and three support modules for the shared builders, fake stores and fake child calls, whose helpers are now public; the other Stage 12 tests import them from those modules, every test keeps its name and body, and two parametrized cases are named after the renamed bundle helper