Enable ruff structure and import rules (INP, PLC, SLF, N, A, FAST, E)

[anth-volk]

Fixes #3919

Summary

Phase 1 step 1.8, group 7: structure and imports. Stacked on #3915.

• Test exemption (decided for this group): test files now also ignore SLF001, INP001 and PLC0415, alongside S101, D1, ANN and PLR2004. Tests deliberately reach into private members, pytest test directories aren't packages, and tests often import inside a test after patching the environment. Application code still gets every rule. api.quality.tests-ruff-exemptions is updated to match.
• Rules: select gains A, E, FAST, INP, N, PLC and SLF. All 289 findings left after the exemption are fixed or kept with an inline reason.
• FastAPI dependencies (FAST002, 21): ruff's own fix would break the v2 routes, which use from __future__ import annotations: FastAPI resolves string annotations against module globals, and the query dependencies were local closures. Each dependency is now a module-level alias instead, such as HouseholdCreateQueryDependency = Annotated[HouseholdCreateQuery, Depends(query_dependency(HouseholdCreateQuery))]. The OpenAPI is byte-identical.
• Imports inside functions (PLC0415, 51):
  • Moved (8): imports that add no cycle, side effect or third-party load. A fresh-interpreter check of all 220 policyengine_api modules shows that policyengine_api.asgi, policyengine_api.api and 211 others load exactly the same modules. Seven route and dependency modules now also load the small, dependency-free readiness and fastapi_routes.dependencies modules.
  • Kept (43), each with a reason: heavy imports (PolicyEngine.py, country models, spm_calculator), Google clients, the v2 persistence stack (data.v2.database registers an exit hook at import), import-time I/O (simulation_entrypoint, specification, constants), one cycle, and two places where tests patch the name per call.
• Namespace packages (INP001, 45): policyengine_api has no __init__.py on purpose, and .github, dashboard/experiments, migrations/v1, migrations/v2 and rules/fixtures aren't packages. They're declared in ruff's top-level namespace-packages setting, with a comment. Compared with the full ruff check output before the change, the only other difference is a hyphenated .github script name (N999), kept with a reason because a shell script runs it by that path. No __init__.py was added, and the built wheel and sdist contain the same files.
• Names (N, A, SLF001): internal names are renamed: two constants in economy_comparison.py, the rules tool's BaseUnavailableError and assert_violation, SQLAlchemy's TimeoutError imported as PoolTimeoutError in tests, and 13 test function names that alone overflowed the line. Kept with a reason: country.py's class reform, since policyengine-core uses the class name as the reform key; two gRPC-stub Export methods in a test; and three private accesses with no public equivalent that tests patch.
• Line length (E501, 149): wrapped. Every split string has identical text, which a syntax-tree comparison confirms. The two published Field descriptions that were split are unchanged. Kept with a reason (6 findings, 4 suppressions): a dotted module path, two GPT-4 prompts in the dashboard experiment, and a docstring line that is a script's --help text.
• Registry: api.quality.ruff-lint now lists the seven families. The shared rules tool changed (wrapped lines and the renames above), so its hash is now f7e9b6462761; sim takes the same copy in Enable ruff structure and import rules (INP, PLC, SLF, N, A, FAST, E) policyengine-sim-api#799.

Testing

• pytest tests/unit tests/contract, to_refactor and the dedupe integration test (the make test set): 2,420 passed, the same as the base. That run was before the stack was rebased onto the release commit; this branch differs from it only in that commit's CHANGELOG.md, pyproject.toml version, uv.lock and changelog fragment.
• make typecheck-v2 is clean. mypy on the 65 changed files outside that gate gives the same output as the base.
• ruff check and ruff format --check (0.15.12) are clean repo-wide.
• Every Pydantic model schema, the FastAPI OpenAPI with every native router enabled, the /v2/openapi.json preview and the legacy spec are byte-identical to the base, with and without OLD_SIMULATION_GATEWAY_URL set.
• The quality guards, the rules guard and the rules tool unittest (75 tests) pass.
• CI/CD: no workflow changed. .github/find-api-model-versions.py gained only a suppression comment.
• CI: this PR is above the bottom of its stack, so it runs only CI complete, which fails and says why. Its full run starts once the PRs below it merge.

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