Enable ruff docstring rules (D)

[anth-volk]

Fixes #778

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 4: docstrings. Stacked on #777.

• Docstrings: ruff.toml gains D and ignores D203 and D213, keeping D211 (no blank line before a class docstring) and D212 (summary on the first line). Every module, class and function outside tests is documented, one commit per project. The 1,043 findings at the base were: executor 427, contract 179, gateway 139, fastapi 118, entry 60, persistence 25, observability 22, integration tests 17, and 56 in repo scripts, proof fixtures and package markers.
• Code is unchanged apart from docstrings. An AST comparison with the base, ignoring docstrings, finds two files with other changes, both equivalent:
  • observability/emitters.py: documented no-op methods drop their redundant return None instead of suppressing RET501.
  • opentelemetry/gcp.py: two misplaced string literals (a class description after __init__, and a function description after its imports) are replaced by docstrings in the right place.
• Suppressions: none for D. An earlier commit suppressed D101 on 61 published contract, ping and health models; the commit "Document published contract models and routes" replaces every one with a docstring.
• Published schema text changes (agreed): Pydantic copies a class docstring into the model's JSON-schema description, and FastAPI copies a route docstring into the operation description. So the entry, gateway and executor OpenAPI documents gain descriptions, written for API consumers. The gateway golden spec (tests/golden/openapi.json) is regenerated with generate_openapi. Compared with the base, ignoring description keys, it's structurally identical: 25 descriptions added, none changed or removed.
• Shared rules tool: both repos documented scripts/rules independently, so the copies drifted. This PR takes the Public API's wording, so the tool is byte-identical again (tool sha256 db83d5ad2aa4 in both rendered rules pages). Only docstrings differ from the previous copy.
• Registry: sim.quality.ruff-lint now lists D.
• After rebasing onto main: Count households once: sum the weight column unweighted #767 added two docstrings whose summary lines aren't in the imperative mood, which D401 requires. The commit "Put the docstrings Count households once: sum the weight column unweighted #767 added in the imperative mood" rewords those two lines.

Testing

• All seven projects' test suites pass:

  Project	Passed
  fastapi	14
  contract	145
  observability	30
  stage12	12
  entry	224
  gateway	214
  executor	733

• 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.

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