fix(plugin-cloud-storage): pass req to find in getFilePrefix for transaction support

[Gdhanush-13]

What

Fixes #15989 - getFilePrefix returns empty prefix during create transaction, causing crop/save flow to fail with Failed to persist upload data.

Root Cause

In packages/plugin-cloud-storage/src/utilities/getFilePrefix.ts, the req.payload.find() call on line 42 does not pass req. During a create operation, the document is being created within a transaction. Without req, the find query runs outside the transaction context and cannot see the uncommitted document, so it returns no results and the prefix resolves to an empty string ''.

Symptoms (as reported)

• Failed to persist upload data for collection ... (warning in afterChange)
• GET /api/media/file/<filename> returns 404
• Crop/edit flow fails during initial upload when using S3 storage with a non-empty prefix

Fix

Added req to the find() call so the query participates in the same transaction and can see the just-created document:

 const files = await req.payload.find({
   collection: collection.slug,
   depth: 0,
   draft: true,
   limit: 1,
   pagination: false,
+  req,
   where: {