fix(vision): il guasto del worker resta suo, il gate anticipa il worker

I2: _call_worker lasciava propagare httpx.ConnectError e ReadTimeout
(worker fermo o job oltre i 120s), senza handler in main.py: l'operatore
vedeva un 500 nudo. Ora ConnectError->502, ReadTimeout->504, ciascuno con
un messaggio che nomina il guasto. La forma della risposta è validata
alla frontiera: outputs/failures/engine_version mancanti o un body non-
dict non fanno più KeyError, rispondono 502 "malformed".

M3: il gate del fuori tolleranza (pending_authorisation) girava dopo
_call_worker: un operatore bloccato bruciava un'intera esecuzione di
visione prima di ricevere il 409. Spostato prima della chiamata al
worker; le quote necessarie erano già caricate prima, nessun'altra query
serviva.

Copertura: test_vision_worker_transport.py (nuovo) per i quattro casi di
guasto/malformazione; test_vision_execute.py aggiunge
test_a_pending_fail_blocks_before_the_worker_is_called, rossa prima dello
spostamento del gate.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014BBnuACZSCJqXrMYC3LUMU

src/backend/tests/test_vision_execute.py

@@ -250,6 +250,45 @@ async def test_a_pending_fail_outside_the_batch_still_blocks(
     )).scalars().all() == []
 
 
+@pytest.mark.asyncio
+async def test_a_pending_fail_blocks_before_the_worker_is_called(
+    client, db_session, measurement_tec_user, monkeypatch,
+):
+    """M3: a blocked operator must not burn a whole vision execution to learn
+    they are blocked. The gate is checked before the worker runs, not after."""
+    recipe = await create_test_recipe(db_session, measurement_tec_user.id)
+    version, task, first, second = await two_vision_quotes(db_session, recipe.id)
+
+    third = RecipeSubtask(
+        task_id=task.id, marker_number=3, description="Not a vision quote",
+        nominal=10.0, utl=10.5, uwl=10.3, lwl=9.7, ltl=9.5, unit="mm",
+    )
+    db_session.add(third)
+    await db_session.commit()
+    await db_session.refresh(third)
+    await measurement_service.save_measurement(
+        db_session, subtask_id=third.id, version_id=version.id,
+        measured_by=measurement_tec_user.id, value=OUT,
+    )
+    await db_session.commit()
+
+    async def worker_must_not_be_called(image_bytes: bytes, graph: dict) -> dict:
+        raise AssertionError("the worker must not run while the operator is blocked")
+    monkeypatch.setattr(
+        "src.backend.services.vision_service._call_worker", worker_must_not_be_called,
+    )
+
+    response = await client.post(
+        "/api/vision/execute",
+        headers=auth_headers(measurement_tec_user),
+        files={"image": ("p.png", an_image(), "image/png")},
+        data={"task_id": str(task.id)},
+    )
+
+    assert response.status_code == 409
+    assert "supervisor" in response.json()["detail"].lower()
+
+
 @pytest.mark.asyncio
 async def test_a_new_acquisition_can_remeasure_its_own_pending_quote(
     client, db_session, measurement_tec_user, worker_says,