fix: add virus scanning to WorkspaceManager.write_file() (#11990)

## Summary

Adds virus scanning at the `WorkspaceManager.write_file()` layer for
defense in depth.

## Problem

Previously, virus scanning was only performed at entry points:
- `store_media_file()` in `backend/util/file.py`
- `WriteWorkspaceFileTool` in
`backend/api/features/chat/tools/workspace_files.py`

This created a trust boundary where any new caller of
`WorkspaceManager.write_file()` would need to remember to scan first.

## Solution

Add `scan_content_safe()` call directly in
`WorkspaceManager.write_file()` before persisting to storage. This
ensures all content is scanned regardless of the caller.

## Changes

- Added import for `scan_content_safe` from `backend.util.virus_scanner`
- Added virus scan call after file size validation, before storage

## Testing

Existing tests should pass. The scan is a no-op in test environments
where ClamAV isn't running.

Closes https://linear.app/autogpt/issue/OPEN-2993

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Introduces a new required async scan step in the workspace write path,
which can add latency or cause new failures if the scanner/ClamAV is
misconfigured or unavailable.
> 
> **Overview**
> Adds a **defense-in-depth** virus scan to
`WorkspaceManager.write_file()` by invoking `scan_content_safe()` after
file-size validation and before any storage/database persistence.
> 
> This centralizes scanning so any caller writing workspace files gets
the same malware check without relying on upstream entry points to
remember to scan.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
0f5ac68b92. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

autogpt_platform/backend/backend/util/workspace.py

@@ -22,6 +22,7 @@ from backend.data.workspace import (
     soft_delete_workspace_file,
 )
 from backend.util.settings import Config
+from backend.util.virus_scanner import scan_content_safe
 from backend.util.workspace_storage import compute_file_checksum, get_workspace_storage
 
 logger = logging.getLogger(__name__)
@@ -187,6 +188,9 @@ class WorkspaceManager:
                 f"{Config().max_file_size_mb}MB limit"
             )
 
+        # Virus scan content before persisting (defense in depth)
+        await scan_content_safe(content, filename=filename)
+
         # Determine path with session scoping
         if path is None:
             path = f"/{filename}"