Cover named helper packages: selector, tableprinter, root, configutil

[shreemaan-abhishek]

Per #14's 2026-04-28 boundary correction, future low-coverage work stays limited to self-contained unit tests on pure helpers, not mocked Admin-API command tests.

Targets:

• pkg/selector
• pkg/tableprinter
• pkg/cmd/root
• pkg/cmd/config/configutil

Also add invalid-parameter branch coverage for service + route + plugin-config combined scenarios (within the Admin-API boundary, not data-plane).

No blanket coverage % target; each named package gets a per-package target set during the work.

Part of #33

[shreemaan-abhishek]

not really needed for GA