s3tables: further refinements to filer operations and utilities

- Add multi-segment namespace support to ARN parsing
- Refactor permission checking to use map lookup
- Wrap lookup errors with ErrNotFound in filer operations
- Standardize splitPath to use path package

weed/s3api/s3tables/filer_ops.go

@@ -82,6 +82,10 @@ func (h *S3TablesHandler) getExtendedAttribute(ctx context.Context, client filer
 		return nil, fmt.Errorf("%w: %s", ErrNotFound, path)
 	}
 
+	if resp.Entry.Extended == nil {
+		return nil, fmt.Errorf("%w: %s", ErrAttributeNotFound, key)
+	}
+
 	data, ok := resp.Entry.Extended[key]
 	if !ok {
 		return nil, fmt.Errorf("%w: %s", ErrAttributeNotFound, key)