Add tests for important edge cases:
- Nested children (child-in-child): verifies that archives within
archives produce correct multi-level AccessPath chains like
"/archive1.tar.gz:/inner-archive.zip:/deep-file.txt"
- Empty child resolver: no locations leak from empty archives
- Same file path in parent and child: both versions accessible
with distinct FileSystemID routing
- Context cancellation in AllLocations: no hanging
Signed-off-by: Thomas Bechtold <thomasbechtold@jpberlin.de>