refactor(strutil): move formatResetAt to strutil.FormatResetAt

The formatResetAt helper in server/circuit_breaker.go already delegates
to strutil.FormatDuration for the relative portion of the output. Moving
it to strutil keeps all time-formatting utilities together, improves
discoverability, and removes a private function from a domain-specific
file.

- Add FormatResetAt(t time.Time) string to internal/strutil/format_duration.go
- Add TestFormatResetAt table-driven tests
- Replace all four call sites in circuit_breaker.go with strutil.FormatResetAt
- Remove the now-unused private formatResetAt function

Addresses: #5032 (Finding 4)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

internal/server/circuit_breaker.go

@@ -195,12 +195,12 @@ func (cb *circuitBreaker) RecordRateLimit(resetAt time.Time) {
 			cb.openedAt = cb.nowFunc()
 			logger.LogError("backend",
 				"circuit breaker for server %q OPENED after %d consecutive rate-limit errors; resets at %s",
-				cb.serverID, cb.consecutiveErrors, formatResetAt(cb.resetAt))
+				cb.serverID, cb.consecutiveErrors, strutil.FormatResetAt(cb.resetAt))
 			logCircuitBreaker.Printf("server %q circuit breaker CLOSED → OPEN (errors=%d)", cb.serverID, cb.consecutiveErrors)
 		} else {
 			logger.LogWarn("backend",
 				"rate-limit error for server %q (consecutive=%d/%d); resets at %s",
-				cb.serverID, cb.consecutiveErrors, cb.threshold, formatResetAt(cb.resetAt))
+				cb.serverID, cb.consecutiveErrors, cb.threshold, strutil.FormatResetAt(cb.resetAt))
 		}
 
 	case circuitHalfOpen:
@@ -209,13 +209,13 @@ func (cb *circuitBreaker) RecordRateLimit(resetAt time.Time) {
 		cb.openedAt = cb.nowFunc()
 		logger.LogError("backend",
 			"circuit breaker for server %q re-OPENED after probe was rate-limited; resets at %s",
-			cb.serverID, formatResetAt(cb.resetAt))
+			cb.serverID, strutil.FormatResetAt(cb.resetAt))
 		logCircuitBreaker.Printf("server %q circuit breaker HALF-OPEN → OPEN (probe rate-limited)", cb.serverID)
 
 	case circuitOpen:
 		// Already open — update reset time.
 		logger.LogWarn("backend", "server %q circuit breaker still OPEN; resets at %s",
-			cb.serverID, formatResetAt(cb.resetAt))
+			cb.serverID, strutil.FormatResetAt(cb.resetAt))
 	}
 }
 
@@ -226,13 +226,6 @@ func (cb *circuitBreaker) State() circuitBreakerState {
 	return cb.state
 }
 
-// formatResetAt returns a human-readable representation of a reset time.
-func formatResetAt(t time.Time) string {
-	if t.IsZero() {
-		return "unknown"
-	}
-	return fmt.Sprintf("%s (in %s)", t.UTC().Format(time.RFC3339), strutil.FormatDuration(time.Until(t).Round(time.Second)))
-}
 
 // buildCircuitBreakers creates per-backend circuit breakers from the configuration.
 func buildCircuitBreakers(cfg *config.Config) map[string]*circuitBreaker {

internal/strutil/format_duration.go

@@ -5,6 +5,16 @@ import (
 	"time"
 )
 
+// FormatResetAt returns a human-readable representation of a reset time,
+// combining an RFC3339 timestamp with a relative countdown (e.g. "2026-05-03T12:00:00Z (in 5.0m)").
+// Returns "unknown" when t is the zero value.
+func FormatResetAt(t time.Time) string {
+	if t.IsZero() {
+		return "unknown"
+	}
+	return fmt.Sprintf("%s (in %s)", t.UTC().Format(time.RFC3339), FormatDuration(time.Until(t).Round(time.Second)))
+}
+
 // FormatDuration formats a duration for display like the debug npm package.
 // It provides granular formatting from nanoseconds to hours.
 func FormatDuration(d time.Duration) string {

internal/strutil/format_duration_test.go

@@ -164,3 +164,18 @@ func TestFormatDuration(t *testing.T) {
 		})
 	}
 }
+
+func TestFormatResetAt(t *testing.T) {
+t.Run("zero time returns unknown", func(t *testing.T) {
+result := FormatResetAt(time.Time{})
+assert.Equal(t, "unknown", result)
+})
+
+t.Run("non-zero time includes RFC3339 timestamp and relative countdown", func(t *testing.T) {
+// Use a fixed future time so the test is deterministic.
+future := time.Date(2030, 1, 1, 12, 0, 0, 0, time.UTC)
+result := FormatResetAt(future)
+assert.Contains(t, result, "2030-01-01T12:00:00Z")
+assert.Contains(t, result, " (in ")
+})
+}