Follow-up to PR #91357 (salvaged, author enwaiax): the committed #90428
explicit-target regression fixture started foreground B with a valid active
runtime and a positive B->runtime cache entry, so routedSessionNeedsResume was
false and the formerly broken foreground-recovery branch was never exercised —
the test passed even on the broken head (b9df1f9c2).
Strengthen it per the review: B now starts with activeSessionIdRef null and an
empty ownership cache, resumeStoredSession(B) fully publishes B's runtime and
cache binding, and the assertions still require no high-level resume of B, an
authoritative session.resume(C), exactly one queued prompt.submit to C's
recovered runtime, and no mutation of foreground refs/cache.
Salvaged-from: PR #91357 (author enwaiax); fixture hardening by maintainer.
Teknium
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