fix(handler): 药品目录按分页契约修正 YPML00001（T-211）

- DrugQuery.PageNo 去 omitempty（pageNo=0 也上送）+ 加 PageSize
- handler Drugs 改 pageNo 必填（parseRequiredInt，缺则返回 400）+ 读 pageSize
- 补缺 pageNo 返回 400 的测试
- 依 docx；YPML00001 未真实联调，pageNo 是否真必填待厂家样本核对

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

handler/public.go

@@ -56,14 +56,19 @@ func (h *PublicHandler) Drugs(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	q := r.URL.Query()
-	pageNo, ok := parseOptionalInt(w, q.Get("pageNo"), "pageNo")
+	pageNo, ok := parseRequiredInt(w, q.Get("pageNo"), "pageNo") // docx: 药品目录是分页查询，pageNo 必填
+	if !ok {
+		return
+	}
+	pageSize, ok := parseOptionalInt(w, q.Get("pageSize"), "pageSize")
 	if !ok {
 		return
 	}
 	_, result, err := h.client.QueryDrugs(r.Context(), osi.DrugQuery{
-		PageNo: pageNo,
-		YPMC:   q.Get("ypmc"),
-		PYM:    q.Get("pym"),
+		PageNo:   pageNo,
+		PageSize: pageSize,
+		YPMC:     q.Get("ypmc"),
+		PYM:      q.Get("pym"),
 	})
 	writeRawOrError(w, result, err)
 }
@@ -82,6 +87,14 @@ func (h *PublicHandler) Orgs(w http.ResponseWriter, r *http.Request) {
 	writeRawOrError(w, result, err)
 }
 
+func parseRequiredInt(w http.ResponseWriter, raw string, name string) (int, bool) {
+	if raw == "" {
+		writeJSONError(w, http.StatusBadRequest, name+" is required")
+		return 0, false
+	}
+	return parseOptionalInt(w, raw, name)
+}
+
 func parseOptionalInt(w http.ResponseWriter, raw string, name string) (int, bool) {
 	if raw == "" {
 		return 0, true