fix(webui): match the health detail row color to its badge, add tests

ProvidersView.vue's accordion detail row hardcoded text-destructive red
for any non-closed health state, while the trigger row's Badge
deliberately used grey (secondary) for half-open and red (destructive)
only for open. Add healthDetailClass so the detail row's color always
matches the badge's own severity, instead of overstating a half-open
probe as an error.

Add a describe block for formatUntil to format.spec.ts, covering the
unset sentinel, undefined, a future timestamp, and a past one. It had
no test coverage since being added in the previous round.

webui/src/lib/format.spec.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 
-import { formatCompactCount, formatContextWindow, formatModelCostHover, refreshDetailLabel, refreshLabel } from './format'
+import { formatCompactCount, formatContextWindow, formatModelCostHover, formatUntil, refreshDetailLabel, refreshLabel } from './format'
 
 const ZERO_TIME = '0001-01-01T00:00:00Z'
 
@@ -98,6 +98,30 @@ describe('formatModelCostHover', () => {
   })
 })
 
+// Discovery circuit breaker (feat/provider-health, round 3): openUntil's
+// future-time counterpart to formatAgo's past-time "(Ns ago)". Only
+// caller today is ProvidersView.vue's healthBadgeLabel, rendering the
+// breaker's live "open (in Ns)" countdown.
+describe('formatUntil', () => {
+  it('renders "" for the unset zero-time sentinel', () => {
+    expect(formatUntil(ZERO_TIME)).toBe('')
+  })
+
+  it('renders "" for undefined', () => {
+    expect(formatUntil(undefined)).toBe('')
+  })
+
+  it('renders "in Ns" for a future timestamp', () => {
+    const inFortyTwoSeconds = new Date(Date.now() + 42_000).toISOString()
+    expect(formatUntil(inFortyTwoSeconds)).toMatch(/^in \d+s$/)
+  })
+
+  it('renders "" for a timestamp already in the past — a stale poll must not show a negative countdown', () => {
+    const oneMinuteAgo = new Date(Date.now() - 60_000).toISOString()
+    expect(formatUntil(oneMinuteAgo)).toBe('')
+  })
+})
+
 // Usage-count compact display (v0.23 addendum): SI-style, 3 significant
 // figures, floor-not-round (the same honesty rule formatRatePercent
 // already applies to the success-rate badge — a count must never appear