When host.agents() rejects (e.g. a registered WSL backend's localhost
forwarding went down), the roster query's catch path returned a
local-only snapshot, so every previously painted remote Bot row vanished
from Bot Mode on the next 5s refresh even though the connection stayed
registered (#98844).
Merge the last-known $lastRoster rows through the same
mergeMultiSourceRoster path used for connect-on-demand sources, marking
carried remote rows sourceReachable: false so they paint as offline
instead of disappearing. sources is intentionally omitted from the
fallback payload so the pane keeps its remembered source snapshot.
Empty-previous behavior is unchanged (local-only list stands).
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