Interpret paths without scheme to be file in DataVaultService (#8761)

URIs with no scheme should be assumed to be file. This already worked in
some circumstances because we do some normalization via
Path.of(stuff).toUri, which adds the file scheme, but this doesn’t
happen in all cases.

### Steps to test:
- Create dataset with an attachment specified with an absolute path,
e.g.
```
            "attachments": {
                "agglomerates": [
                    { "name": "agglomerate_view_70", "dataFormat": "zarr3", "path": "/home/agglomerate_view_70" }
                ]
            },
```
- Agglomerate mapping should be applied correctly.

### Issues:
- fixes
https://scm.slack.com/archives/C5AKLAV0B/p1751900152180879?thread_ts=1751888599.160149&cid=C5AKLAV0B
- follow-up refactoring:
#8762

------
- [x] Added changelog entry (create a `$PR_NUMBER.md` file in
`unreleased_changes` or use `./tools/create-changelog-entry.py`)
- [x] Considered [common edge
cases](../blob/master/.github/common_edge_cases.md)
- [x] Needs datastore update after deployment

Author: fm3

unreleased_changes/8761.md

@@ -0,0 +1,2 @@
+### Fixed
+- Fixed a bug where dataset layer attachments with absolute paths would not get loaded unless the paths are specified with file:// scheme.

webknossos-datastore/app/com/scalableminds/webknossos/datastore/datavault/FileSystemDataVault.scala

@@ -94,7 +94,7 @@ class FileSystemDataVault extends DataVault {
   private def vaultPathToLocalPath(path: VaultPath)(implicit ec: ExecutionContext): Fox[Path] = {
     val uri = path.toUri
     for {
-      _ <- Fox.fromBool(uri.getScheme == DataVaultService.schemeFile) ?~> "trying to read from FileSystemDataVault, but uri scheme is not file"
+      _ <- Fox.fromBool(uri.getScheme == null || uri.getScheme == DataVaultService.schemeFile) ?~> "trying to read from FileSystemDataVault, but uri scheme is not file or null"
       _ <- Fox.fromBool(uri.getHost == null || uri.getHost.isEmpty) ?~> s"trying to read from FileSystemDataVault, but hostname ${uri.getHost} is non-empty"
       localPath = Paths.get(uri.getPath)
       _ <- Fox.fromBool(localPath.isAbsolute) ?~> "trying to read from FileSystemDataVault, but hostname is non-empty"

webknossos-datastore/app/com/scalableminds/webknossos/datastore/models/datasource/DatasetLayerAttachments.scala

@@ -3,6 +3,7 @@ package com.scalableminds.webknossos.datastore.models.datasource
 import com.scalableminds.util.enumeration.ExtendedEnumeration
 import com.scalableminds.util.io.PathUtils
 import com.scalableminds.util.tools.{Box, Full}
+import com.scalableminds.webknossos.datastore.storage.DataVaultService
 import org.apache.commons.io.FilenameUtils
 import play.api.libs.json.{Format, Json}
 
@@ -41,7 +42,7 @@ case class LayerAttachment(name: String,
                            credentialId: Option[String] = None) {
   // Warning: throws! Use inside of tryo
   def localPath: Path = {
-    if (path.getScheme.nonEmpty && path.getScheme != "file") {
+    if (path.getScheme != null && path.getScheme.nonEmpty && path.getScheme != DataVaultService.schemeFile) {
       throw new Exception(
         "Trying to open non-local hdf5 file. Hdf5 files are only supported on the datastore-local file system.")
     }

webknossos-datastore/app/com/scalableminds/webknossos/datastore/storage/DataVaultService.scala

@@ -55,7 +55,7 @@ class DataVaultService @Inject()(ws: WSClient, config: DataStoreConfig) extends
         S3DataVault.create(remoteSource, ws)
       } else if (scheme == DataVaultService.schemeHttps || scheme == DataVaultService.schemeHttp) {
         HttpsDataVault.create(remoteSource, ws, config.Http.uri)
-      } else if (scheme == DataVaultService.schemeFile) {
+      } else if (scheme == DataVaultService.schemeFile || scheme == null) {
         FileSystemDataVault.create
       } else {
         throw new Exception(s"Unknown file system scheme $scheme")