feat(FN-4041): harden empty rawResponse handling in memory-insights

Hardened empty `rawResponse` handling in the memory insights module by adding defensive checks to prevent errors when the response is empty, accompanied by tests to cover those edge cases (FN-4041 Step 1).

Fusion-Task-Id: FN-4041

packages/core/src/__tests__/memory-insights.test.ts

@@ -838,6 +838,19 @@ describe("memory-insights run processing", () => {
       expect(result.summary).toContain("AI timeout");
     });
 
+    it("should treat undefined successful response as no output", async () => {
+      const result = await processInsightExtractionRun(tempDir, {
+        rawResponse: undefined,
+        stepSuccess: true,
+        runAt: new Date().toISOString(),
+      });
+
+      expect(result.insights).toHaveLength(0);
+      expect(result.summary).toBe("Step did not produce output");
+      expect(result.newInsightCount).toBe(0);
+      expect(result.duplicateCount).toBe(0);
+    });
+
     it("should preserve existing insights on failure", async () => {
       // Create existing insights
       const existingInsights = `# Memory Insights

packages/core/src/memory-insights.ts

@@ -230,7 +230,7 @@ interface MemoryAuditState {
 /** Input for processing an insight extraction run. */
 export interface ProcessRunInput {
   /** Raw AI response text from the insight extraction step. */
-  rawResponse: string;
+  rawResponse?: string;
   /** Whether the AI step itself succeeded. */
   stepSuccess: boolean;
   /** Timestamp of the run. */
@@ -1118,7 +1118,7 @@ export async function processInsightExtractionRun(
   let parseError: string | undefined;
 
   // Try to parse the AI response
-  if (stepSuccess && rawResponse.trim()) {
+  if (stepSuccess && rawResponse?.trim()) {
     try {
       parsedResult = parseInsightExtractionResponse(rawResponse);
     } catch (err) {