fix(confidence-engine): sanitize reasoning error responses

tests/focused-deconstruct-boundary.test.js

@@ -360,7 +360,7 @@ describe("focused-deconstruct targetNodeId identity boundary", () => {
     expect(json.providerExecution).toBeUndefined();
   });
 
-  it("preserves the 500 provider-failure contract while logging structural diagnostics", async () => {
+  it("sanitizes generic provider failures while logging structural diagnostics", async () => {
     const errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
     vi.doMock("@/lib/llm/provider", () => ({
       getProvider: () => ({
@@ -383,7 +383,7 @@ describe("focused-deconstruct targetNodeId identity boundary", () => {
         }),
       }));
       expect(response.status).toBe(500);
-      await expect(response.json()).resolves.toEqual({ error: "provider failed" });
+      await expect(response.json()).resolves.toEqual({ error: "Reasoning request could not be completed." });
       expect(errorSpy).toHaveBeenCalledWith(
         "[api/focused-investigation/deconstruct] provider failure",
         expect.objectContaining({ targetNodeId: "node-id", providerApiPath: "/v1/responses" }),