## Summary
Fix file uploads in AI chat (Agent Chat) failing with "Failed to upload
file" for all file types.
## Root Cause
`useAIChatFileUpload.ts` was using `useApolloCoreClient()` which points
to the `/graphql` (workspace) endpoint. However, `FileUploadResolver` is
decorated with `@MetadataResolver()`, meaning `uploadFile` is only
registered on the `/metadata` endpoint.
## Fix
Replace `useApolloCoreClient()` with `useApolloClient()` from
`@apollo/client`, which is the default Apollo client pointing to the
`/metadata` endpoint. This is consistent with how
`useUploadAttachmentFile.tsx` handles file uploads.
## Changes
- Replaced `useApolloCoreClient` import with `useApolloClient` from
`@apollo/client`
- Updated the client variable to use the default Apollo client
- Removed unused `useApolloCoreClient` import
Fixes#18153
