_live_session_payload() falls back to _fallback_session_info() while a
session's agent is still None (lazy/deferred build). That fallback omitted
desktop_contract, so session.activate returned lazy metadata with no contract
field. Desktop feeds the value straight into reportBackendContract(), where a
missing field reads as contract 0 — a current backend is then falsely flagged
"Backend out of date" on every activate of a live lazy session.
The sibling session.create shape (_lazy_resume_info) was fixed the same way in
#36112; this closes the remaining session.activate gap by advertising
DESKTOP_BACKEND_CONTRACT in the fallback payload.
Adds test_session_activate_lazy_info_reports_desktop_contract pinning the
session.activate path against a lazy (agent=None) session.
a1da384c6d