vision: anteprima DXF nel worker e nel backend (Refs #101)

Worker: POST /dxf/preview legge il DXF con read_xld_dxf (lo stesso lettore di
CompareDxf) e ritorna contorni in mm con y verso il basso, bbox, dimensioni,
entita e punti (tetto 20000, decimazione uniforme). Backend: POST
/api/vision/dxf-preview (Maker) che inoltra al worker; 422 inoltrato.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/vision_worker/tests/test_dxf_compare.py

@@ -80,3 +80,67 @@ async def test_dxf_compare_immagine_non_decodificabile_e_422():
 async def test_dxf_compare_parametro_fuori_range_e_422():
     response = await _post(_png(a_scene(0.0)), SAMPLE_DXF.read_bytes(), polarity="grigio")
     assert response.status_code == 422
+
+
+# --- #101: POST /dxf/preview ---------------------------------------------------
+
+def _rect_dxf(width: float, height: float, y0: float = 3.0) -> bytes:
+    """DXF minimo con ezdxf: un rettangolo chiuso, y in alto rispetto a 0."""
+    import ezdxf
+    doc = ezdxf.new()
+    doc.modelspace().add_lwpolyline(
+        [(1, y0), (1 + width, y0), (1 + width, y0 + height), (1, y0 + height)], close=True,
+    )
+    buffer = io.StringIO()
+    doc.write(buffer)
+    return buffer.getvalue().encode("utf-8")
+
+
+def _post_preview(dxf: bytes):
+    # TestClient sincrono: non serve pytest-asyncio (assente nel venv del worker)
+    from fastapi.testclient import TestClient
+    return TestClient(app).post(
+        "/dxf/preview", files={"dxf": ("d.dxf", dxf, "application/octet-stream")},
+    )
+
+
+def test_dxf_preview_rettangolo_10x5():
+    resp = _post_preview(_rect_dxf(10, 5))
+    assert resp.status_code == 200, resp.text
+    body = resp.json()
+    assert body["larghezza_mm"] == 10 and body["altezza_mm"] == 5
+    assert body["entita"] == 1 and body["punti"] == sum(len(c) for c in body["contorni"])
+    # y ribaltata: il rettangolo stava a y 3..8 nel DXF, ora a -8..-3 (asse verso il basso)
+    assert body["bbox"] == {"xmin": 1, "ymin": -8, "xmax": 11, "ymax": -3}
+    ring = body["contorni"][0]
+    assert ring[0] == ring[-1]  # contorno chiuso: primo == ultimo
+    assert all(-8 <= y <= -3 for _, y in ring)
+
+
+@pytest.mark.parametrize("content", [b"", b"questo non e' un dxf"])
+def test_dxf_preview_illeggibile_e_422(content):
+    resp = _post_preview(content)
+    assert resp.status_code == 422
+    assert resp.json()["detail"].startswith("DXF")
+
+
+def test_dxf_preview_senza_geometria_e_422():
+    import ezdxf
+    doc = ezdxf.new()
+    buffer = io.StringIO()
+    doc.write(buffer)
+    resp = _post_preview(buffer.getvalue().encode("utf-8"))
+    assert resp.status_code == 422
+    assert "senza geometria" in resp.json()["detail"]
+
+
+def test_dxf_preview_decima_oltre_il_tetto(monkeypatch):
+    from src.vision_worker import main
+    monkeypatch.setattr(main, "_PREVIEW_MAX_POINTS", 50)
+    import ezdxf
+    doc = ezdxf.new()
+    doc.modelspace().add_circle((0, 0), 20)
+    buffer = io.StringIO()
+    doc.write(buffer)
+    full = main._dxf_preview(buffer.getvalue().encode("utf-8"))
+    assert full["punti"] <= 50 + 1 and full["larghezza_mm"] == pytest.approx(40, abs=0.01)  # dimensioni dai punti completi