refactor: extract PROCEDURAL_TREE_TYPES constant and add negative tree_type test

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/tests/test_maintenance_tree_type.py

@@ -50,3 +50,24 @@ async def test_list_maintenance_trees_filter(client: AsyncClient, auth_headers:
     trees = resp.json()
     assert all(t["tree_type"] == "maintenance" for t in trees)
     assert len(trees) >= 1
+
+
+@pytest.mark.asyncio
+async def test_invalid_tree_type_rejected(client: AsyncClient, auth_headers: dict):
+    """An unrecognized tree_type value is rejected with 422."""
+    resp = await client.post(
+        "/api/v1/trees",
+        json={
+            "name": "Bad Type",
+            "tree_type": "garbage",
+            "tree_structure": {
+                "steps": [
+                    {"id": "s1", "type": "procedure_step", "title": "Step",
+                     "description": "Do it", "content_type": "action"},
+                    {"id": "end", "type": "procedure_end", "title": "Done"},
+                ]
+            },
+        },
+        headers=auth_headers,
+    )
+    assert resp.status_code == 422