fix(FileUploaderDropContainer): enforce multiple prop on drag-and-drop

[adamalston]

Closes #18668

Enforced the multiple prop on drag-and-drop in FileUploaderDropContainer.

Changelog

Changed

• Enforced the multiple prop on drag-and-drop in FileUploaderDropContainer.
• Updated tests that were uploading multiple files and expecting them to all be processed without the multiple prop.

Testing / Reviewing

yarn test packages/react/src/components/FileUploader

[netlify]

✅ Deploy Preview for v11-carbon-web-components ready!

Name	Link
🔨 Latest commit	3890588
🔍 Latest deploy log	https://app.netlify.com/sites/v11-carbon-web-components/deploys/67b8ecb0faa5b50008670414
😎 Deploy Preview	https://deploy-preview-18669--v11-carbon-web-components.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[netlify]

✅ Deploy Preview for carbon-elements ready!

Name	Link
🔨 Latest commit	3890588
🔍 Latest deploy log	https://app.netlify.com/sites/carbon-elements/deploys/67b8ecb0f7bb0000088925f0
😎 Deploy Preview	https://deploy-preview-18669--carbon-elements.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[netlify]

✅ Deploy Preview for v11-carbon-react ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	3890588
🔍 Latest deploy log	https://app.netlify.com/sites/v11-carbon-react/deploys/67b8ecb0472bdf0008aaccde
😎 Deploy Preview	https://deploy-preview-18669--v11-carbon-react.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.97%. Comparing base (b67877b) to head (3890588).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #18669   +/-   ##
=======================================
  Coverage   84.97%   84.97%           
=======================================
  Files         408      408           
  Lines       14542    14544    +2     
  Branches     4764     4736   -28     
=======================================
+ Hits        12357    12359    +2     
+ Misses       2024     2023    -1     
- Partials      161      162    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.