fix: include DiskType in metadata log volume assignment (#7918)

When writing metadata logs to /topics/.system/log, the filer was not
respecting the disk type configuration from path-specific rules
(fs.configure). This caused volume assignment failures when volume
servers used a specific disk type (e.g., "ssd") because the assign
request defaulted to empty disk type.

The fix adds DiskType to the VolumeAssignRequest in the filer's
metadata log write path, ensuring that path-specific disk type
configurations are properly honored for internal system writes.

Fixes errors like:
  "metadata log write failed /topics/.system/log/...: AssignVolume:
   failed to find writable volumes for collection"

Signed-off-by: Charles Darke <s.cduk@toodevious.com>
Co-authored-by: Charles Darke <s.cduk@toodevious.com>

weed/filer/filer_notify_append.go

@@ -55,6 +55,7 @@ func (f *Filer) assignAndUpload(targetFile string, data []byte) (*operation.Assi
 		Count:               1,
 		Collection:          util.Nvl(f.metaLogCollection, rule.Collection),
 		Replication:         util.Nvl(f.metaLogReplication, rule.Replication),
+		DiskType:            rule.DiskType,
 		WritableVolumeCount: rule.VolumeGrowthCount,
 	}