fix: return 404 (not 502) when function Service is gone (DNS no such host)

After trigger/function destroy the Service is deleted, DNS lookup fails with
'no such host'. Previously InvokeFunction returned 502 which kept the test
script retrying for 120s. Now returns 404 — endpoint removed, test passes.

Operator: naeel/sless-operator:v0.1.18

internal/api/handler/invoke.go

@@ -59,6 +59,12 @@ func (h *Handler) InvokeFunction(w http.ResponseWriter, r *http.Request) {
 
 	resp, err := httpClient.Do(proxyReq)
 	if err != nil {
+		// "no such host" — Service не существует (функция удалена), возвращаем 404.
+		// Это отличается от временной сетевой ошибки: NXDOMAIN строго означает отсутствие записи.
+		if strings.Contains(err.Error(), "no such host") {
+			writeJSON(w, http.StatusNotFound, errResp("function not found"))
+			return
+		}
 		h.Log.Error("invoke: function unreachable", "err", err, "ns", ns, "fn", name, "target", target)
 		writeJSON(w, http.StatusBadGateway, errResp("function unreachable: "+err.Error()))
 		return