fix(media,canvas): 对象存储参考素材优先 URL，并同步清理图片引用提示词 (#434)

* fix(media): 对象存储参考素材 - 协议支持 URL 时优先发送签名链接

开启对象存储后，支持 URL 的上游协议优先发送 OSS/COS/七牛/S3 签名链接；必须传字节的协议仍走 Base64 或 multipart。本地资源回退字节，签发失败直接报错，不自动重发生成。

* fix(canvas): 图片引用清理 - 取消引用时同步移除提示词引用

web/test/canvas-node-generation-mentions.test.ts

@@ -93,4 +93,16 @@ describe("canvas node generation position mentions", () => {
         expect(context.prompt).toBe("让 @图片1 进入画面");
         expect(context.prompt).not.toContain("@[node:");
     });
+
+    test("取消引用后的提示词不再因为悬挂 @图片1 阻断生成", () => {
+        const target = targetNode();
+        const image = node("image-a", CanvasNodeType.Image, "data:image/png;base64,a");
+        const { applyCanvasConnectionPromptSync } = require("../src/lib/canvas/canvas-resource-references") as typeof import("../src/lib/canvas/canvas-resource-references");
+        const [nextTarget] = applyCanvasConnectionPromptSync([image, target], [connection(image.id)], [image, target], []).filter((item: typeof target) => item.id === target.id);
+        const context = buildNodeGenerationContext(nextTarget.id, [image, nextTarget], [], nextTarget.metadata?.composerContent || "", []);
+
+        expect(nextTarget.metadata?.composerContent).toBe("让 进入画面");
+        expect(context.referenceImages).toEqual([]);
+        expect(context.prompt).toBe("让 进入画面");
+    });
 });