experiment(confidence-engine): route UI journey provider centrally

tests/app/api/current-understanding-synthesis-route.test.js

@@ -10,6 +10,7 @@ vi.mock("@/lib/graph/current-understanding-synthesis.js", () => ({
 
 vi.mock("@/lib/llm/provider.js", () => ({
   getProvider: () => ({}),
+  getProviderModelName: () => "gpt-5.6-terra",
 }));
 
 // ── Helpers ─────────────────────────────────────────────────
@@ -45,6 +46,7 @@ describe("POST /api/cases/synthesis — valid request", () => {
     expect(data.success).toBe(true);
     expect(data.currentUnderstanding).toBe("Synthesized result");
     expect(mockSynthesize).toHaveBeenCalledTimes(1);
+    expect(mockSynthesize.mock.calls[0][1]).toMatchObject({ modelName: "gpt-5.6-terra" });
   });
 
   it("returns narrative result on success", async () => {