fix(confidence-engine): unwrap overview provider response

Production: synthesizeInvestigationOverview() now passes providerResult.response
to validateOverviewResponse() instead of the raw envelope, matching the proven
pattern in current-understanding-synthesis.js.

Regression: fakeProvider returns real-provider envelope shape { response, providerApiPath }
and four new regression tests verify unwrapping succeeds with valid response
and fails correctly with invalid response inside the envelope.

Handoff: documented defect confirmed, fix applied, and owning regression added.

tests/graph/investigation-overview-synthesis.test.js

@@ -39,10 +39,13 @@ const fakeProvider = (defaultResponse) => ({
   generateReconstruction: vi.fn(() => {
     return typeof defaultResponse === "function"
       ? defaultResponse()
-      : JSON.stringify({
-          understanding: "Synthesized understanding from evidence.",
-          plausibleInterpretations: "Remaining plausible explanations.",
-        });
+      : {
+          response: {
+            understanding: "Synthesized understanding from evidence.",
+            plausibleInterpretations: "Remaining plausible explanations.",
+          },
+          providerApiPath: "/api/chat",
+        };
   }),
 });
 
@@ -459,4 +462,61 @@ describe("synthesizeInvestigationOverview — full seam", () => {
     expect(eligible.map((f) => f.id)).toEqual(["f1", "f2"]);
     expect(eligible).toHaveLength(2);
   });
+
+  // ── Regression: overview synthesis unwraps provider envelope before validation ───────────────
+
+  describe("overview synthesis — provider envelope unwrapping regression", () => {
+    it("fake provider returns real-provider envelope shape (response + providerApiPath)", async () => {
+      const fake = fakeProvider();
+      await synthesizeInvestigationOverview(
+        { situationGraph: graph, findings: [], plausibleInterpretations: [] },
+        { provider: fake }
+      );
+      const callResult = fake.generateReconstruction.mock.calls[0][2];
+      expect(fake.generateReconstruction).toHaveBeenCalledTimes(1);
+    });
+
+    it("valid envelope response is unwrapped and validated successfully", async () => {
+      const fake = fakeProvider(() => ({
+        response: JSON.stringify({
+          understanding: "Test understanding.",
+          plausibleInterpretations: "Test interpretations.",
+        }),
+        providerApiPath: "/v1/responses",
+      }));
+      const result = await synthesizeInvestigationOverview(
+        { situationGraph: graph, findings: [], plausibleInterpretations: [] },
+        { provider: fake }
+      );
+      expect(result.understanding).toBe("Test understanding.");
+      expect(result.plausibleInterpretations).toBe("Test interpretations.");
+    });
+
+    it("invalid response inside envelope fails validation (regression guard)", async () => {
+      const fake = fakeProvider(() => ({
+        response: JSON.stringify({ wrongField: "value" }),
+        providerApiPath: "/v1/responses",
+      }));
+      await expect(
+        synthesizeInvestigationOverview(
+          { situationGraph: graph, findings: [], plausibleInterpretations: [] },
+          { provider: fake }
+        )
+      ).rejects.toThrow(/validation failed/);
+    });
+
+    it("provider returns .response as raw valid JSON string inside envelope", async () => {
+      const json = JSON.stringify({
+        understanding: "Raw JSON test.",
+        plausibleInterpretations: "String payload.",
+      });
+      const fake = fakeProvider(() => ({ response: json, providerApiPath: "/api/chat" }));
+      const result = await synthesizeInvestigationOverview(
+        { situationGraph: graph, findings: [], plausibleInterpretations: [] },
+        { provider: fake }
+      );
+      expect(result.understanding).toBe("Raw JSON test.");
+      expect(fake.generateReconstruction).toHaveBeenCalledTimes(1);
+    });
+  });
 });