fix(zapier): harden null-body handling and error messages

From an independent review pass:
- Guard search / get / add against a null or empty 2xx body (previously an
  uncaught TypeError on .results / .status); return [] or an empty object.
- Stringify non-string API error detail so a thrown message never renders as
  "[object Object]".
- Correct the Add sample to the default no-wait shape {event_id, status}.
- Add a regression unit test for the null-body path.

Verified: build + zapier validate + 16 tests (13 unit, 3 live E2E), plus a
live check of the v3-add to /v1/event poll cross-version path.

integrations/zapier-mem0/src/searches/search_memories.ts

@@ -16,9 +16,9 @@ const perform = async (z: ZObject, bundle: Bundle): Promise<Memory[]> => {
 		method: 'POST',
 		body,
 	});
-	// Searches must return an array.
-	const data = response.data as Memory[] | { results?: Memory[] };
-	return Array.isArray(data) ? data : data.results || [];
+	// Searches must return an array; guard against a null/empty body.
+	const data = response.data as Memory[] | { results?: Memory[] } | null;
+	return Array.isArray(data) ? data : data?.results ?? [];
 };
 
 export default {