Enable ruff docstring rules (D)

[anth-volk]

Fixes #3906

Summary

Phase 1 step 1.8, group 4: docstrings. Stacked on #3905.

• Docstrings: select gains D and ignores D203 and D213, keeping D211 (no blank line before a class docstring) and D212 (summary on the first line). All 1,336 findings at the base are fixed: about 1,000 docstrings added or reworded across 229 Python files, committed by area (runtime cache and libraries, rules tool and scripts, data layer, v2 services, FastAPI routes and models, Flask routes and utilities, v1 services). Existing docstrings were reworded to say what the code actually does; one test docstring said 404 where the code returns 400.
• Code is unchanged apart from docstrings. An AST comparison with the base, ignoring docstrings, pass and ..., finds one other change, which is equivalent. In full:
  • 27 classes whose body was only pass now have a docstring instead, because PIE790 flags pass after a docstring.
  • Protocol stubs keep ... after their new docstring.
  • The disabled cache backend's get() drops its redundant return None instead of suppressing RET501.
• Suppressions: no noqa: D… anywhere. The two validator proof fixtures for api.v2.validators-transformations-no-db get one-line docstrings rather than a per-file ignore.
• 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. Compared with the base, with description keys removed:
  • Structure is identical: the same paths, operations, properties, types and required lists in all 234 model schemas, the FastAPI OpenAPI (every native router enabled) and the /v2/openapi.json preview.
  • Published OpenAPI: 57 component descriptions added, written for API consumers. No field or operation descriptions changed.
  • Model schemas: 128 models gained or changed their own description (127 new; EconomicImpactResult reworded).
  • Legacy spec: byte-identical. No checked-in schema snapshot needed regenerating.
• Shared rules tool: scripts/rules is byte-identical with sim again (tool sha256 db83d5ad2aa4); sim adopts this PR's wording in Enable ruff docstring rules (D) policyengine-sim-api#779.
• Registry: api.quality.ruff-lint now lists D.

Testing

• pytest tests/unit tests/contract, to_refactor and the dedupe integration test (the make test set): 2,411 passed.
• mypy (configured scope) is clean. ruff check and ruff format --check (0.15.12) are clean repo-wide.
• Quality guards, the rules guard and the rules tool unittest pass.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 24.32432% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.77%. Comparing base (ca36072) to head (376029d).

Files with missing lines	Patch %	Lines
policyengine_api/fastapi_routes/dependencies.py	9.75%	37 Missing ⚠️
policyengine_api/runtime_cache/core.py	50.00%	14 Missing ⚠️
policyengine_api/api.py	0.00%	1 Missing ⚠️
policyengine_api/services/household_mirroring.py	0.00%	1 Missing ⚠️
policyengine_api/services/policy_mirroring.py	0.00%	1 Missing ⚠️
policyengine_api/services/user_policy_mirroring.py	0.00%	1 Missing ⚠️
policyengine_api/worker_spm.py	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (ca36072) and HEAD (376029d). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (ca36072)	HEAD (376029d)
	2	1

Additional details and impacted files

@@                  Coverage Diff                  @@
##           ruff-annotations    #3907       +/-   ##
=====================================================
- Coverage             85.38%   43.77%   -41.61%     
=====================================================
  Files                   198      183       -15     
  Lines                 12400    11284     -1116     
  Branches               2264     1954      -310     
=====================================================
- Hits                  10588     4940     -5648     
- Misses                 1111     5893     +4782     
+ Partials                701      451      -250     

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