feat(endpoint): include 422 validation-error response in endpoint namespace

[yeongseon]

Context

The endpoint metadata namespace (_endpoint.py, shipped via #275 under umbrella #270) currently emits a responses map that contains only the success status (success_status_code -> response object). It does not describe the standardized 422 validation-error response that @validate_http actually returns at runtime ({"detail": [{"loc", "msg", "type"}]}, see pipeline.py:222 and README "Status codes and controlled errors").

As a result, azure-functions-openapi cannot document the 422 contract from the shared namespace, so generated specs omit the error response even though runtime behavior is fixed and well-defined. This is the still-open P1 item "Validation error response schema 반영" from the v0.8 code review.

Note: this supersedes the stale #35, which was closed against the pre-endpoint-namespace design (enable_openapi_integration()), an API that no longer exists.

Target: keep ENDPOINT_METADATA_VERSION at 1 — this is an additive change to the responses map (consumers read defensively with .get()), no schema version bump.

Acceptance Checklist

• build_endpoint_metadata adds a "422" entry to responses whenever any request model (body/query/path/headers) is present, using a self-contained JSON Schema for the {"detail": [...]} envelope (no model classes, no $ref leakage beyond the SPEC-pinned template).
• The 422 schema matches the runtime envelope exactly (loc, msg, type item keys) — cross-checked against the golden tests in test_public_api.py / test_integration.py.
• schemas/endpoint.schema.json + docs/METADATA_SPEC.md updated to document the 422 response shape; endpoint.schema.sha256 regenerated.
• New unit test asserting the endpoint namespace responses contains both the success status and "422" with the correct schema.
• Coverage stays >= 95% (make cov); make check-all green.
• Translated READMEs updated if any English doc changes (per AGENTS.md).

Out of scope

• Consumer-side rendering of the 422 response in azure-functions-openapi (tracked separately in that repo; see feat(metadata): converge cross-package endpoint metadata without new foundation packages #270 checklist).
• Custom ErrorFormatter output schemas — the emitted 422 documents the default envelope only.

References

• Umbrella: feat(metadata): converge cross-package endpoint metadata without new foundation packages #270
• Superseded stale design: Add deeper azure-functions-openapi integration (422 schema) #35
• Producer: src/azure_functions_validation/_endpoint.py (build_endpoint_metadata, responses)
• Runtime envelope: src/azure_functions_validation/pipeline.py:222, README "Status codes and controlled errors"