fix(ai): risposta AI malformata -> 502, non 400 (#36 punto 7)

json.JSONDecodeError e' un ValueError: KeyError/JSONDecodeError sulla
risposta del modello finivano nell'except ValueError del router (pensato
per i nostri errori di configurazione, es. API key mancante) e uscivano
come 400 invece di 502. Nuova AIResponseError dedicata in ai_service.py
per contenuto malformato o JSON non valido; recipes.py la traduce in 502,
toccato solo per questo except.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/tests/test_ai_service.py

@@ -0,0 +1,67 @@
+"""Tests for ai_service.py error handling (gitea#36 point 7).
+
+A malformed response from the AI model (missing fields, invalid JSON) must
+raise the dedicated AIResponseError, not a bare KeyError/JSONDecodeError —
+JSONDecodeError in particular is itself a ValueError subclass, so without
+this it was caught by the router's `except ValueError` (meant for our own
+config errors) and misreported as a 400 instead of a 502.
+"""
+import json
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from src.backend.services import ai_service
+
+
+class _FakeResponse:
+    def __init__(self, json_data):
+        self._json_data = json_data
+
+    def raise_for_status(self):
+        pass
+
+    def json(self):
+        return self._json_data
+
+
+def _patch_openrouter(monkeypatch, response_json: dict):
+    monkeypatch.setattr(ai_service, "extract_text_from_pdf", lambda _b: "some extracted text")
+    monkeypatch.setattr(ai_service.settings, "openrouter_api_key", "test-key")
+    monkeypatch.setattr(ai_service.settings, "openrouter_model", "test-model")
+
+    fake_client = AsyncMock()
+    fake_client.post = AsyncMock(return_value=_FakeResponse(response_json))
+    fake_client.__aenter__ = AsyncMock(return_value=fake_client)
+    fake_client.__aexit__ = AsyncMock(return_value=False)
+
+    return patch("src.backend.services.ai_service.httpx.AsyncClient", return_value=fake_client)
+
+
+async def test_missing_choices_key_raises_ai_response_error(monkeypatch):
+    with _patch_openrouter(monkeypatch, {"no_choices_here": True}):
+        with pytest.raises(ai_service.AIResponseError):
+            await ai_service.parse_technical_sheet(b"%PDF-fake")
+
+
+async def test_invalid_json_content_raises_ai_response_error(monkeypatch):
+    bad_content = "not valid json {{{"
+    response_json = {"choices": [{"message": {"content": bad_content}}]}
+    with _patch_openrouter(monkeypatch, response_json):
+        with pytest.raises(ai_service.AIResponseError):
+            await ai_service.parse_technical_sheet(b"%PDF-fake")
+
+
+async def test_valid_json_content_returns_tasks(monkeypatch):
+    tasks = [{"title": "MATERIALI", "directive": "Verificare", "description": "..."}]
+    response_json = {"choices": [{"message": {"content": json.dumps(tasks)}}]}
+    with _patch_openrouter(monkeypatch, response_json):
+        result = await ai_service.parse_technical_sheet(b"%PDF-fake")
+    assert result == tasks
+
+
+async def test_empty_extracted_text_returns_empty_list(monkeypatch):
+    """No AI call at all when the PDF has no extractable text."""
+    monkeypatch.setattr(ai_service, "extract_text_from_pdf", lambda _b: "   ")
+    result = await ai_service.parse_technical_sheet(b"%PDF-fake")
+    assert result == []