fix(mnemopi): preserve empty structured extraction

Short-circuit successfully parsed structured extractor output even when all extraction arrays are empty, so the JSON response body is not stored as a fallback memory.

Added coverage for plain and fenced empty extraction JSON.

Fixes #4390

packages/mnemopi/src/core/extraction.ts

@@ -115,9 +115,7 @@ export function parseFacts(rawOutput: string | null | undefined): string[] {
 						}
 					}
 				}
-				if (out.length > 0) {
-					return out.slice(0, 5);
-				}
+				return out.slice(0, 5);
 			}
 		} catch {
 			const matches = [...raw.matchAll(/"([^"]{10,})"/g)].map(m => m[1]).filter((v): v is string => v !== undefined);

packages/mnemopi/test/extraction.test.ts

@@ -52,6 +52,13 @@ describe("structured extraction", () => {
 		expect(parseFacts("NO_FACTS")).toEqual([]);
 	});
 
+	it("treats a valid empty structured extraction as no facts", () => {
+		expect(parseFacts('{"facts": [], "instructions": [], "preferences": [], "timelines": [], "kg": []}')).toEqual([]);
+		expect(
+			parseFacts('```json\n{"facts": [], "instructions": [], "preferences": [], "timelines": [], "kg": []}\n```'),
+		).toEqual([]);
+	});
+
 	it("uses deterministic heuristic extraction when no LLM is configured", async () => {
 		process.env.MNEMOPI_LLM_ENABLED = "false";
 		const facts = await extractFactsSafe("My name is Ada. I work at Example Corp and I prefer dark mode.");