fix: move agent attachment download api (#15146)

### What problem does this PR solve?

move agent attachment download api to the correct route and update
frontend callers

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

### Notes

- Move the attachment download endpoint from document routes to agent
routes.
- Update frontend download callers to use the agent attachment endpoint.
- Reuse the shared file response header helper instead of duplicating it
in `agent_api.py`.

web/src/utils/api.ts

@@ -129,8 +129,6 @@ export default {
     `${restAPIv1}/datasets/${datasetId}/documents/${documentId}`,
   documentThumbnails: `${restAPIv1}/thumbnails`,
   getDocumentFile: `${restAPIv1}/documents`,
-  getDocumentFileDownload: (docId: string) =>
-    `${restAPIv1}/documents/${docId}/download`,
   documentUpload: (datasetId: string) =>
     `${restAPIv1}/datasets/${datasetId}/documents`,
   webCrawl: (datasetId: string) =>
@@ -223,6 +221,8 @@ export default {
     `${restAPIv1}/agentbots/${canvasId}/inputs`,
   prompt: `${restAPIv1}/agents/prompts`,
   cancelDataflow: (id: string) => `${restAPIv1}/tasks/${id}/cancel`,
+  getAttachmentFileDownload: (docId: string) =>
+    `${restAPIv1}/agents/${docId}/download`,
   downloadFile: `${restAPIv1}/agents/download`,
   testWebhook: (id: string) => `${restAPIv1}/agents/${id}/webhook/test`,
   fetchWebhookTrace: (id: string) => `${restAPIv1}/agents/${id}/webhook/logs`,