test(backend): factorise les doubles de session

Chaque test reecrivait sa classe de session et sa fonction d'override, soit
trois fois le meme decor pour un seul endpoint. FakeSession et la fixture
fake_session portent ce decor, make_settings fabrique une Settings dont les
valeurs priment sur l'environnement.

apps/backend/tests/api/test_health.py

@@ -1,12 +1,9 @@
-from collections.abc import AsyncIterator
+from collections.abc import Callable
 
 import pytest
-from fastapi import FastAPI
 from httpx import AsyncClient
 from sqlalchemy.exc import OperationalError
 
-from app.db.session import get_session
-
 
 async def test_liveness_exposes_service_metadata(client: AsyncClient) -> None:
     response = await client.get("/api/v1/health/live")
@@ -20,15 +17,10 @@ async def test_liveness_exposes_service_metadata(client: AsyncClient) -> None:
     }
 
 
-async def test_readiness_reports_the_timescaledb_version(app: FastAPI, client: AsyncClient) -> None:
-    class ReadySession:
-        async def scalar(self, *_: object, **__: object) -> str:
-            return "2.22.1"
-
-    async def override() -> AsyncIterator[ReadySession]:
-        yield ReadySession()
-
-    app.dependency_overrides[get_session] = override
+async def test_readiness_reports_the_timescaledb_version(
+    fake_session: Callable[..., None], client: AsyncClient
+) -> None:
+    fake_session(result="2.22.1")
 
     response = await client.get("/api/v1/health/ready")
 
@@ -41,16 +33,9 @@ async def test_readiness_reports_the_timescaledb_version(app: FastAPI, client: A
 
 
 async def test_readiness_returns_503_when_the_extension_is_missing(
-    app: FastAPI, client: AsyncClient
+    fake_session: Callable[..., None], client: AsyncClient
 ) -> None:
-    class SessionWithoutExtension:
-        async def scalar(self, *_: object, **__: object) -> None:
-            return None
-
-    async def override() -> AsyncIterator[SessionWithoutExtension]:
-        yield SessionWithoutExtension()
-
-    app.dependency_overrides[get_session] = override
+    fake_session(result=None)
 
     response = await client.get("/api/v1/health/ready")
 
@@ -67,16 +52,9 @@ async def test_readiness_returns_503_when_the_extension_is_missing(
     ids=["erreur_sqlalchemy", "erreur_reseau_asyncpg"],
 )
 async def test_readiness_returns_503_when_database_is_unreachable(
-    app: FastAPI, client: AsyncClient, failure: Exception
+    fake_session: Callable[..., None], client: AsyncClient, failure: Exception
 ) -> None:
-    class UnreachableSession:
-        async def scalar(self, *_: object, **__: object) -> None:
-            raise failure
-
-    async def override() -> AsyncIterator[UnreachableSession]:
-        yield UnreachableSession()
-
-    app.dependency_overrides[get_session] = override
+    fake_session(failure=failure)
 
     response = await client.get("/api/v1/health/ready")