fix(storage-s3): support S3 client uploads over 5 GiB via multipart

[codyparnell]

Which branch?

Targets main (Payload v4).

What?

Fixes S3 client uploads failing for files larger than 5 GiB.
AWS S3 limits single PutObject uploads to 5 GiB. This PR ensures the S3 client upload handler automatically switches to multipart upload for larger files, and documents the related Payload config requirements.

Why?

With clientUploads: true, files upload directly from the browser to S3. That bypasses server/Vercel body-size limits, but two constraints still apply:

1. S3 single-request limit (5 GiB) — files above this cannot use a single presigned PutObject URL.
2. Payload upload.limits.fileSize — the signed-URL signer still enforces this limit for both single-part and multipart flows.
   Without multipart routing and clear configuration guidance, large client uploads fail (e.g. Exceeded file size limit on initiateMultipart or generateSignedURL).

How?

Client (S3ClientUploadHandler)

• Files ≤ 5 GiB → existing single-part flow (generateSignedURL → S3 PUT)
• Files > 5 GiB → multipart flow:
  • initiateMultipart
  • concurrent part uploads with retry
  • completeMultipart (with abortMultipart on failure)

Server (generateSignedURL handler)

• Adds multipart signer actions: initiateMultipart, signMultipartPart, completeMultipart, abortMultipart
• Rejects single-part signed URLs above 5 GiB with a clear error directing callers to multipart
• Treats upload.limits.fileSize: Infinity as no signer-side cap
• Prefixes size-limit errors with the action name (e.g. initiateMultipart: Exceeded file size limit) for easier debugging

Docs & tests

• Documents client upload limits, the 5 GiB threshold, and upload.limits.fileSize in packages/storage-s3/README.md
• Integration tests cover multipart initiate/complete/abort and related edge cases in test/storage-s3/client-uploads/

User-facing config requirement

Users uploading large files must raise the global limit to match their largest expected file:

export default buildConfig({
  upload: {
    limits: {
      fileSize: 7 * 1024 * 1024 * 1024, // e.g. 7 GiB
      // or Infinity to disable the signer-side cap
    },
  },
})