feat(frontend): Add description to Upload Agent dialog (#11053)

### Changes 🏗️

- Added a description to the Upload Agent dialog to provide more context
for users. Fixes
[BUILDER-3N1](https://sentry.io/organizations/significant-gravitas/issues/6915512912/).
The issue was that: DialogContent in LibraryUploadAgentDialog lacks an
accessible description, violating WAI-ARIA standards.

<img width="2066" height="1740" alt="image"
src="https://github.com/user-attachments/assets/c876fb33-4375-4a66-a6a2-6b13c00ef8d3"
/>


### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  <!-- Put your test plan here: -->
  - [x] Test it works
  - [x] Get design approval

Co-authored-by: seer-by-sentry[bot] <157164994+seer-by-sentry[bot]@users.noreply.github.com>
Co-authored-by: Nicholas Tindle <[email protected]>

Author: seer-by-sentry[bot]

autogpt_platform/frontend/src/app/(platform)/library/components/LibraryUploadAgentDialog/LibraryUploadAgentDialog.tsx

@@ -4,6 +4,7 @@ import { Button } from "@/components/__legacy__/Button";
 import {
   Dialog,
   DialogContent,
+  DialogDescription,
   DialogHeader,
   DialogTitle,
   DialogTrigger,
@@ -64,6 +65,9 @@ export default function LibraryUploadAgentDialog(): React.ReactNode {
       <DialogContent>
         <DialogHeader>
           <DialogTitle className="mb-8 text-center">Upload Agent</DialogTitle>
+          <DialogDescription>
+            Upload your agent by providing a name, description, and JSON file.
+          </DialogDescription>
         </DialogHeader>
 
         <Form {...form}>