fix: surface video filename errors during upload

[santhosh-apphelix-2u]

Summary

When a video filename contains non-ASCII characters, the backend returns a specific validation error. Studio was displaying only the generic Failed to add... message.

Changes

• Pass the backend error message through the video upload thunk.
• Preserve the message in the Redux error state.
• Return immediately after a resolved non-2xx response to prevent a follow-up generic error.
• Add tests for both resolved error responses and rejected Axios requests.

Expected behavior

Users now see the backend message with the actual filename, for example:

The file name for lecture – intro.mp4 must contain only ASCII characters.