Add optional `?prefix=` query string support to the existing
`/api/:collection/file/:filename` endpoint to disambiguate files
that share the same filename but have different storage prefixes.

When a storage prefix (e.g., UUID or folder path) is used to make
S3 keys unique, multiple documents can share the same `filename`.
The current endpoint resolves by filename alone, which can match
the wrong document in both access control and file retrieval.

- `checkFileAccess` accepts optional `prefix` and adds it to the
  `where` clause alongside existing access filters
- `getFile` handler reads `prefix` from `req.searchParams` and
  threads it to `checkFileAccess` and handler `params`
- `getFilePrefix` accepts `explicitPrefix` to skip the DB query
  when the prefix is already known from the URL
- S3 `staticHandler` forwards `prefix` from params to `getFilePrefix`
- `afterRead` hook appends `?prefix=` to Payload-proxied URLs
- Added unit tests for `checkFileAccess` and `getFilePrefix`,
  and integration tests for the endpoint'

When a media URL already contains query parameters, appending the
imageCacheTag with '?' produces a malformed URL with double '?'.
Check for existing query parameters and use '&' as the separator
when needed, matching the pattern already used in the Thumbnail
component.