The B4 fix made listDirectory require a JSON content-type (to detect the
SPA-fallback HTML / 502 cases) and changed the non-OK error string, but its
tests still mocked headerless responses + the old message, so they failed —
which also polluted the run and tripped AppIconGrid's teardown. Give the JSON
mock a content-type, update the non-OK expectation, and add a test for the
guard's friendly-error path. Full suite now 667/667 green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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