fix: resolve thumbnailURL to direct cloud storage URLs

[thedanielmark]

What?

Fixes #12659

When using cloud storage with adminThumbnail set to a string size name (for example 'thumbnail'), thumbnailURL could resolve to /api/{collection}/file/... instead of the direct CDN or S3 URL. This affected admin list thumbnails and any API response that relied on thumbnailURL.

Why?

Two independent bugs in core:

1. appendUploadSelectFields
   For string adminThumbnail, the list-view select only projected sizes.<name>.filename, not .url. Without the stored URL in the projection, the thumbnailURL afterRead hook falls back to building a local API path from the filename.

2. generateFilePathOrURL
   When serverURL is unset, which is common for relative-URL deployments, urlOrPath.startsWith(serverURL || '') becomes startsWith(''). That is always true, so external URLs were treated as same-origin and discarded in favor of the local file path.

Related: #15988 adds generateFileURL hooks on thumbnailURL in plugin-cloud-storage. That work is complementary. It regenerates URLs on read when generateFileURL is configured. This PR fixes select projection and external URL detection in core.

How?

• Add url: true to the sizes.<adminThumbnail> select in appendUploadSelectFields, matching the behavior of the non-string adminThumbnail branch.
• In generateFilePathOrURL, only treat urlOrPath as same-origin when serverURL is actually configured.

Verification

• pnpm test:unit appendUploadSelectFields generateFilePathOrURL
• pnpm test:int storage-s3
• pnpm test:int plugin-cloud-storage
• pnpm test:int uploads