feat(l1): advance_ai_build — record answer + generate next node

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_l1_session_service.py

@@ -796,6 +796,79 @@ async def test_start_ai_build_session(test_db: AsyncSession):
     assert s.status == "active"
 
 
+# ---------------------------------------------------------------------------
+# T8: advance_ai_build
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_advance_ai_build_appends_and_returns_next(test_db: AsyncSession, monkeypatch):
+    from app.services import l1_session_service as svc
+    from app.services import ai_tree_builder
+    account = await _make_account(test_db)
+    l1_user = await _make_user(test_db, account_id=account.id)
+    s = await svc.start_ai_build_session(
+        test_db, account_id=account.id, user=l1_user,
+        ticket_id="t-ai", ticket_kind="internal")
+
+    async def fake_next(problem, category, walked):
+        return {"node_type": "resolved", "id": "done", "text": "Fixed."}
+    monkeypatch.setattr(ai_tree_builder, "generate_next_node", fake_next)
+
+    next_node = await svc.advance_ai_build(
+        test_db, session_id=s.id, problem_text="printer", category="printer",
+        node_id="n1", node_text="Powered on?", answer="no", note=None)
+    assert next_node["node_type"] == "resolved"
+    refreshed = await test_db.get(type(s), s.id)
+    assert len(refreshed.walked_path) == 1
+    assert refreshed.walked_path[0]["answer"] == "no"
+    assert refreshed.walked_path[0]["text"] == "Powered on?"
+
+
+@pytest.mark.asyncio
+async def test_advance_ai_build_first_call_does_not_append(test_db: AsyncSession, monkeypatch):
+    from app.services import l1_session_service as svc
+    from app.services import ai_tree_builder
+    account = await _make_account(test_db)
+    l1_user = await _make_user(test_db, account_id=account.id)
+    s = await svc.start_ai_build_session(
+        test_db, account_id=account.id, user=l1_user,
+        ticket_id="t-ai-first", ticket_kind="internal")
+
+    async def fake_next(problem, category, walked):
+        return {"node_type": "question", "id": "q1", "text": "Is it plugged in?"}
+    monkeypatch.setattr(ai_tree_builder, "generate_next_node", fake_next)
+
+    # First call: node_id=None — nothing should be appended
+    next_node = await svc.advance_ai_build(
+        test_db, session_id=s.id, problem_text="printer", category="printer",
+        node_id=None)
+    assert next_node["node_type"] == "question"
+    assert next_node["id"] == "q1"
+    refreshed = await test_db.get(type(s), s.id)
+    assert len(refreshed.walked_path) == 0
+    assert refreshed.current_node_id == "q1"
+
+
+@pytest.mark.asyncio
+async def test_advance_ai_build_wrong_session_kind_raises(test_db: AsyncSession, monkeypatch):
+    from app.services import l1_session_service as svc
+    from app.services import ai_tree_builder
+    account = await _make_account(test_db)
+    l1_user = await _make_user(test_db, account_id=account.id)
+    # start an adhoc session (not ai_build)
+    s = await svc.start_adhoc_session(
+        test_db, account_id=account.id, user=l1_user,
+        ticket_id="t-adhoc-guard", ticket_kind="internal")
+
+    async def fake_next(problem, category, walked):  # pragma: no cover
+        return {"node_type": "question", "id": "q1", "text": "?"}
+    monkeypatch.setattr(ai_tree_builder, "generate_next_node", fake_next)
+
+    with pytest.raises(ValueError, match="ai_build"):
+        await svc.advance_ai_build(
+            test_db, session_id=s.id, problem_text="printer", category="printer")
+
+
 # ---------------------------------------------------------------------------
 # T14 audit log tests (spec §5.6.1)
 # ---------------------------------------------------------------------------