Normalize empty ls/glob tool output for the model.
This middleware exists only because the SDK's ls/glob tools currently
serialize a successful-but-empty result as the literal "[]". Upstream is
expected to start returning useful empty-result content directly, at which
point this middleware becomes redundant and should be removed. The canary
test test_sdk_still_returns_bracket_for_empty_listing in
tests/unit_tests/test_filesystem_empty_result.py fails loudly when that
upstream change lands, signalling that this module can be deleted.
