ci: scoped 100% coverage gates + policy doc

backend/tests/test_error_handling.py

@@ -4,7 +4,9 @@ from fastapi import FastAPI, HTTPException
 from fastapi.testclient import TestClient
 from pydantic import BaseModel, Field
 
-from app.core.error_handling import REQUEST_ID_HEADER, install_error_handling
+from starlette.requests import Request
+
+from app.core.error_handling import REQUEST_ID_HEADER, _error_payload, _get_request_id, install_error_handling
 
 
 def test_request_validation_error_includes_request_id():
@@ -80,3 +82,38 @@ def test_response_validation_error_returns_500_with_request_id():
     assert body["detail"] == "Internal Server Error"
     assert isinstance(body.get("request_id"), str) and body["request_id"]
     assert resp.headers.get(REQUEST_ID_HEADER) == body["request_id"]
+
+
+def test_client_provided_request_id_is_preserved():
+    app = FastAPI()
+    install_error_handling(app)
+
+    @app.get("/needs-int")
+    def needs_int(limit: int) -> dict[str, int]:
+        return {"limit": limit}
+
+    client = TestClient(app)
+    resp = client.get("/needs-int?limit=abc", headers={REQUEST_ID_HEADER: "  req-123  "})
+
+    assert resp.status_code == 422
+    body = resp.json()
+    assert body["request_id"] == "req-123"
+    assert resp.headers.get(REQUEST_ID_HEADER) == "req-123"
+
+
+def test_get_request_id_returns_none_for_missing_or_invalid_state() -> None:
+    # Empty state
+    req = Request({"type": "http", "headers": [], "state": {}})
+    assert _get_request_id(req) is None
+
+    # Non-string request_id
+    req = Request({"type": "http", "headers": [], "state": {"request_id": 123}})
+    assert _get_request_id(req) is None
+
+    # Empty string request_id
+    req = Request({"type": "http", "headers": [], "state": {"request_id": ""}})
+    assert _get_request_id(req) is None
+
+
+def test_error_payload_omits_request_id_when_none() -> None:
+    assert _error_payload(detail="x", request_id=None) == {"detail": "x"}