fix(server): keep a failed usage probe marked when a sparse update lands

[macodev00]

What Changed

Fixes pingdotgg#12170.

applyUsageLimitsUpdate now copies unavailable onto the rebuilt snapshot the same way it already copies resetCredits. A sparse rate_limit_event merged onto probeFailed or onto no previous snapshot stays marked incomplete instead of publishing as a one-window complete list. Unchanged updates keep the previous object (so a probeFailed no-op no longer rebuilds and strips the marker). unsupported is still rejected.

resolveUsageLimitsAfterProbe is unchanged: a snapshot that still has unavailable is not last-good, so a later failed probe does not keep weekly-only as complete. Cache hydration and a partial UI reason are out of scope.

Why

After a failed (or missing) usage probe, Claude's one-window rate_limit_event was upserted onto [] and rebuilt without unavailable. Clients treated that as a full read and drew only the weekly bar; the next failed probe kept it because the marker was gone.

pingdotgg#12294 landed the same merge fix and was closed against the V2 provider rewrite. The hole is still on main; this is the triage's small server bugfix, not a new limits protocol and not blocked on pingdotgg#10597.

UI Changes

No new UI. While the marker stays on, Limits keeps showing "Could not read limits." instead of a false weekly-only card. Screenshots/video not applicable.

Tests

vp test run src/provider/providerUsageLimits.test.ts src/provider/makeManagedServerProvider.test.ts in apps/server: 19 passed, including the new merge/probeFailed regressions and the existing "keeps live usage windows across a failed probe". Lint and format clean on the two files.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

  

[macodev00]

Superseded by upstream pingdotgg#12700