The spinoff endpoint authenticated the caller (_require_user) but never
verified the research session belonged to them before reading the
persisted report and seeding it into a new chat session owned by the
caller. Any authenticated user who knew or guessed another user's
research session ID could exfiltrate that user's full report into their
own session — a cross-user data disclosure (IDOR).
Every other endpoint in this router gates on _owns_in_memory /
_assert_owns_research right after validating the session ID; spinoff was
the lone exception. Add the same _owns_in_memory check (covers both the
in-memory task and the on-disk JSON) so a non-owner gets a 404 before any
data is read or a session is created.
Add regression tests pinning the anonymous (401) and wrong-owner (404)
cases.
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