统一画布代理文本与附件输入约束

要求用户消息包含非空文本，附件仅作为消息上下文。
同步后端领域校验、接口错误和请求测试。
禁用前端纯附件发送并更新交互回归测试。
保留发送失败时的草稿与附件恢复行为。

src/components/image-editor/EditorAgentConversation/EditorAgentConversationPanelView.tsx

@@ -134,6 +134,7 @@ export function EditorAgentConversationPanelView({
     isWaiting ||
     isToolCallActionPending ||
     isPastingAttachment ||
+    !draftText.trim() ||
     !hasProject;
   const currentConversationTitle = activeConversation?.title ?? '新对话';
 
@@ -147,9 +148,6 @@ export function EditorAgentConversationPanelView({
       return;
     }
     const text = draftText.trim();
-    if (!text && !attachments.length) {
-      return;
-    }
     setDraftText('');
     const nextAttachments = consumeAttachments();
     void sendMessage(text, nextAttachments).catch(() => {
@@ -366,10 +364,7 @@ export function EditorAgentConversationPanelView({
             <button
               type="submit"
               className="inline-flex h-10 min-w-16 shrink-0 items-center justify-center gap-1.5 rounded-full bg-slate-900 px-3 text-sm font-semibold text-white disabled:opacity-45"
-              disabled={
-                isMessageSubmissionBlocked ||
-                (!draftText.trim() && !attachments.length)
-              }
+              disabled={isMessageSubmissionBlocked}
             >
               <Send className="h-3.5 w-3.5" aria-hidden="true" />
               发送