fix(ui): prevent re-render cascade in UploadHandlersProvider

[mahmoodhamdi]

What

Fixes the memory leak / browser crash when using multiple collections with a cloud storage plugin (e.g. s3Storage, azureStorage, gcsStorage).

Closes #16039

Why

UploadHandlersProvider used useState to store the handlers map. Each ClientUploadHandler provider (one per upload collection) calls setUploadHandler on mount, which triggers setState → a full re-render of the admin tree below it. With N collections, that's N state updates each causing a cascading re-render of the entire admin panel, plus N Map clone allocations.

The handlers map doesn't need to trigger re-renders — getUploadHandler is only called during form submission, never during rendering.

Changes

• Replaced useState with useRef for the upload handlers map — registering a handler no longer triggers a re-render
• Wrapped getUploadHandler and setUploadHandler in useCallback for stable references
• Memoized the context value with useMemo to prevent unnecessary consumer re-renders

Testing

• pnpm run build:ui passes with no errors
• Verified that getUploadHandler is only consumed during form submission (Form/index.tsx:582) and bulk upload (FormsManager/index.tsx:362), never during render — so switching from state to ref is safe
• The context type signature (UploadHandlersContext) and public API are unchanged