test(catalog): cover P4 VIN group-node gate (children + loadError)

Mirrors the PSA VIN-scope tests for the new …group.action parent gate: a Ford
json-vin-sub-group.action node with a positional link_wid routes to children
(not a silent empty leaf), and an empty drill surfaces loadError.

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

apps/api/src/categories/categories.service.spec.ts

@@ -320,6 +320,84 @@ describe("CategoriesService", () => {
         "drill-load-error",
       );
     });
+
+    it("routes a P4 Ford VIN node (json-vin-sub-group.action, positional link_wid) to children, not an empty leaf", async () => {
+      // link_wid "1" is a positional code (not a *Group* table) so the link_wid
+      // group-drill branch misses it; the path is …group.action (not /psa/, not
+      // vin-group.action) so the PSA / Volvo branches miss it too. Before the
+      // dedicated gate these fell through to the leaf parts path → silent empty
+      // panel (Ford/Nissan/Opel analog of the PSA-parent-gate gap).
+      const category = {
+        id: "ford1",
+        name: "elektrik sistemi",
+        nameOriginal: "elektrik sistemi",
+        parentId: null,
+        vehicleId: "v1",
+        source: "pl24",
+        linkPath: "/ford/fordp_parts/json-vin-sub-group.action?mainGroupId=CAP1&vin=WF0FXX",
+        linkWid: "1",
+        hasSubgroups: null,
+        hasParts: null,
+      };
+      let selectCall = 0;
+      const db = {
+        select: vi.fn().mockImplementation(() => {
+          selectCall++;
+          const captured = selectCall;
+          const c: Record<string, any> = {};
+          c.from = vi.fn().mockReturnValue(c);
+          c.where = vi.fn().mockImplementation(() => (captured === 2 ? [] : c));
+          c.limit = vi.fn().mockReturnValue([category]);
+          return c;
+        }),
+        execute: vi.fn().mockResolvedValue([]),
+      };
+      const { service } = createService(db);
+      const discovered = [{ id: "sg1", name: "kablolar", children: undefined }];
+      vi.spyOn(service, "getChildren").mockResolvedValue(discovered as any);
+
+      const result = await service.getCategoryWithParts("ford1");
+
+      expect(service.getChildren).toHaveBeenCalledWith("ford1");
+      expect(result.parts).toEqual([]);
+      expect((result as { children?: unknown }).children).toEqual(discovered);
+    });
+
+    it("surfaces loadError when a P4 group.action drill comes back empty (not a silent leaf)", async () => {
+      const category = {
+        id: "ford2",
+        name: "kaporta",
+        nameOriginal: "kaporta",
+        parentId: null,
+        vehicleId: "v1",
+        source: "pl24",
+        linkPath: "/ford/fordp_parts/json-vin-sub-group.action?mainGroupId=GDB&vin=WF0FXX",
+        linkWid: "2",
+        hasSubgroups: null,
+        hasParts: null,
+      };
+      let selectCall = 0;
+      const db = {
+        select: vi.fn().mockImplementation(() => {
+          selectCall++;
+          const captured = selectCall;
+          const c: Record<string, any> = {};
+          c.from = vi.fn().mockReturnValue(c);
+          c.where = vi.fn().mockImplementation(() => (captured === 2 ? [] : c));
+          c.limit = vi.fn().mockReturnValue([category]);
+          return c;
+        }),
+        execute: vi.fn().mockResolvedValue([]),
+      };
+      const { service } = createService(db);
+      vi.spyOn(service, "getChildren").mockResolvedValue([] as any);
+
+      const result = await service.getCategoryWithParts("ford2");
+
+      expect(service.getChildren).toHaveBeenCalledWith("ford2");
+      expect((result as { loadError?: boolean }).loadError).toBe(true);
+      expect((result as { children?: unknown }).children).toBeUndefined();
+    });
   });
 
   describe("getAncestors", () => {