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

[anth-volk]

Fixes #798

Same-Repository Draft PR Check

• This PR is a draft.
• This PR is from a branch in PolicyEngine/policyengine-sim-api, not a personal fork.

Summary

Phase 1 step 1.8, group 7: structure and imports. Stacked on #785.

• Test exemption (decided for this group): **/tests/** and **/authenticated_tests/** 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. Both test-exemption rules in the registry are updated to match.
• Rules: ruff.toml gains A, E, FAST, INP, N, PLC and SLF. All 339 findings left after the exemption are fixed or kept with an inline reason.
• Imports inside functions (PLC0415, 176):
  • Moved (16): only imports that load nothing new, create no cycle and aren't patched by tests. Each was checked by importing every affected module in a fresh interpreter.
  • Kept (160), each with a reason:
    • Modal and image code: 56. Function bodies import packages that exist only in the image.
    • Test seams: 52. Tests monkeypatch these names, so they're looked up at call time.
    • Lazy heavy or optional dependencies: 49.
    • Import cycles: 3.
• Namespace packages (INP001, 13): .github/scripts, scripts, the executor's scripts and rules/fixtures aren't packages, so they're declared in ruff's top-level namespace-packages setting. Compared with the full ruff check output before the change, nothing else differs. The executor's src/modal/utils scripts keep an inline suppression instead: declaring that directory made ruff treat import modal (the Modal SDK) as first-party, and an __init__.py would change the source copied into the Modal images.
• Names (N, A, PLC0414, SLF001):
  • Renamed exceptions: the entry app's BackendUnavailableError, BackendTimeoutError and TemporaryStage12UnsupportedRequestError, and the executor's ParityMismatchError. Each is caught by name and never logged or stored by class name.
  • Kept: TemporaryStage12DispatchFailed, because the entry app logs its class name as error_type.
  • Published fields: the 14 camelCase budget-window fields in gateway_models.py keep their names.
  • Other renames: three OpenTelemetry locals, the exit import aliased as app_exit, and four gateway test names shortened for line length.
  • Kept with a reason: the contract's explicit re-exports, the exit object projects import, and two private accesses with no public equivalent.
• FastAPI (FAST001, 13): response_model arguments that repeat the return type are dropped. The gateway golden OpenAPI is unchanged, and the entry, executor and gateway OpenAPI dumps are identical before and after.
• Line length (E501): wrapped, with every split string's text identical, which a syntax-tree comparison confirms. Four multi-line fake shell scripts in tests keep a suppression, because their one-line printf JSON can't be split without changing the script.
• Modal images: many executor and gateway image files change, mostly in import placement and line wrapping; src/modal/app.py gains no __future__ import. CI's image smoke will run when this PR reaches the bottom of the stack.
• Registry: sim.quality.ruff-lint now lists the seven families. The shared rules tool is synced from the Public API's group 7 (Enable ruff structure and import rules (INP, PLC, SLF, N, A, FAST, E) policyengine-api#3920), so it's byte-identical again (tool sha256 f7e9b6462761).

Testing

• All seven projects' test suites pass, with the same counts as the base:

  Project	Passed
  fastapi	14
  contract	145
  observability	30
  stage12	12
  entry	224
  gateway	214
  executor	744

• The gateway golden OpenAPI test passes, and the golden file is unchanged.

• ruff check (0.15.22) is clean repo-wide, and ruff format --check is unchanged.

• The rules guard, the ci-scripts guard and the rules tool unittest pass.

• The integration-test project's conftest change was checked by parsing only; its tests need live services.

• CI/CD: no workflow changed, and no environment variable was added or rewired.

• CI: this PR is above the bottom of its stack, so it runs only CI complete, which fails and says why (step 0.7, Make CI enforcement trustworthy before the rules registry #754). Its full run starts once the PRs below it merge.

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