ClientSessionState.usage is Partial<UsageStats> (app/types.ts) — the
backend streams whichever fields changed — so the computed produces
ReadableAtom<Partial<UsageStats> | null>. Annotate the entry honestly
instead of claiming full UsageStats, and document the fallback rule for
plugin authors. Also collapse the three-argument expect() calls in the
contract test (vitest takes one message arg). Addresses triage review on
PR #80461.
a445a81279