feat(ui): improve admin upload error for Vercel 4.5 MB function body limit

[premsreelathasugeendran]

Summary

Closes #16484. When a Payload admin upload hits HTTP 413, the toast now reports the actual file size and points contributors at the recommended fix (`clientUploads: true` on cloud-storage adapters) instead of the generic "request was too large to submit successfully".

Before / After

Before (static `errorMessages[413]`):

Your request was too large to submit successfully.

After (when a file is present in the FormData — most uploads):

Upload failed: this file is 5.1 MB, but this deployment can only accept about 4.5 MB through the server upload route. Hosted / serverless providers (e.g. Vercel Functions) cap the request body lower than Payload's configured upload.limits.fileSize. For cloud-storage adapters like @payloadcms/storage-vercel-blob, enable clientUploads to bypass this limit, or upload a smaller file.

The 5.1 MB number is computed from the actual file the user just tried to submit.

Files

• `packages/ui/src/forms/Form/errorMessages.ts` — adds `buildPayloadTooLargeMessage` helper + tightens the static fallback string
• `packages/ui/src/forms/Form/index.tsx` — branches on `res.status === 413` and calls the helper

How it works

```ts
const message =
res.status === 413
? buildPayloadTooLargeMessage({ body: formData })
: errorMessages?.[res.status] || res?.statusText || t('error:unknown')
```

The helper scans the submitted `FormData` for the largest `Blob`/`File` and formats a context-rich message; if the body is not FormData (e.g. a JSON request that happens to 413), it falls back to the static map entry — which now also mentions `clientUploads` and the ~4.5 MB Vercel limit so non-FormData 413s still get actionable copy.

Test plan

• No new dependencies; the helper is pure and uses standard `FormData` / `Blob` APIs
• Non-413 errors take exactly the same path as before — diff is additive at the 413 branch only
• Reviewer step: deploy a sandbox to Vercel with `@payloadcms/storage-vercel-blob` and `clientUploads: false`, attempt to upload a 5+ MB file, confirm the new toast renders with the actual size
• Reviewer step: upload a 1 MB file (no 413) and confirm normal success path is unchanged

Notes for review

• Kept the helper next to `errorMessages.ts` (rather than a new file) because it's a tiny presentation helper coupled to the same status-code map.
• Source for the 4.5 MB number: Vercel Functions runtime docs — Request body size limit.
• i18n: matches the existing pattern in this file (English-only, no translation keys). Happy to wire into `@payloadcms/translations` if you'd prefer — wasn't sure if that's overkill for an error string this specific.
• Related prior context: discussion Vercel Blob Storage Plugin client upload only files under 4,5 mb possible to upload #7569 and issue 4.5MB Upload Limit when deployed on Vercel #8231.