fix: preserve canvas agent media metadata

lib/canvas/agent-mutations.ts

@@ -116,6 +116,7 @@ function buildMediaNode(params: {
   const now = new Date().toISOString();
   const type = mediaNodeType(params.item);
   const metadata = {
+    ...(params.plan.metadata || {}),
     ...(params.item.metadata || {}),
     generatedByAgent: true,
     canvasAgentOperationId: params.plan.operationId,

tests/routes/canvas-agent-operations.test.ts

@@ -361,7 +361,21 @@ test.describe("canvas agent operation mutation contract", () => {
         sourceNodeIds: ["image-source"],
         anchorNodeId: "image-source",
         placement: "right",
-        mediaItems: [buildMutationMediaItem("media-revision", ["image-source"])],
+        mediaItems: [
+          {
+            ...buildMutationMediaItem("media-revision", ["image-source"]),
+            prompt: "turn the background white",
+            metadata: {
+              model: "test-image-model",
+              provider: "test-provider",
+            },
+          },
+        ],
+        metadata: {
+          messageId: "assistant-message-1",
+          chatId: "chat-1",
+          model: "plan-level-model",
+        },
       },
     });
 
@@ -376,11 +390,17 @@ test.describe("canvas agent operation mutation contract", () => {
       type: "image",
       data: {
         src: "https://cdn.example.com/media-revision.png",
+        prompt: "turn the background white",
         metadata: {
           generatedByAgent: true,
           canvasAgentOperationId: "op-append-revision",
           canvasAgentMediaItemId: "media-revision",
           revisionOf: "image-source",
+          sourceNodeIds: ["image-source"],
+          messageId: "assistant-message-1",
+          chatId: "chat-1",
+          model: "test-image-model",
+          provider: "test-provider",
         },
       },
     });