test: make the offline resolve check tamper-evident

The test asserted only that resolve() returned one entry. It returns one
entry whether the injected fetch answered or the real transport was
blocked, because resolve() records a transport failure as a per-entry
error rather than raising. So the assertion would have stayed green if
someone removed the injection, which is the one thing the test exists to
notice.

It now asserts the handle that only the injected fetch can produce.
Removing the injection fails with KeyError: 'handle'.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wrfqr2xqQfhtXCscU7zrdz

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