fix: handle missing sandbox backend in test_list_command #116

Job	Run time
Docker Integration Tests	49s
Build (ubuntu-latest, x86_64-unknown-linux-gnu, agentkernel-linux-x64)	1m 5s
Check	19s
Format	14s
Clippy	21s
Test (macos-latest)	1m 4s
Security Audit	2m 37s
Test (ubuntu-latest)	33s
Build (macos-latest, aarch64-apple-darwin, agentkernel-darwin-arm64)	1m 29s
Build (macos-15-intel, x86_64-apple-darwin, agentkernel-darwin-x64)	2m 5s
	10m 36s

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