fix(ratelimit): clear window token counters on reset from response headers

UpdateFromHeaders was silently updating ResetsAt without clearing token
counters. When a window rolled over, the poll method would see ResetsAt
already updated and skip the reset. Extract setResetTime helper used by
both code paths.

internal/ratelimit/tracker.go

@@ -71,7 +71,7 @@ func (t *Tracker) UpdateFromHeaders(h http.Header) {
 	}
 	if v := h.Get("Anthropic-Ratelimit-Unified-5h-Reset"); v != "" {
 		if ts, err := strconv.ParseInt(v, 10, 64); err == nil {
-			t.fiveHour.ResetsAt = time.Unix(ts, 0).UTC().Truncate(time.Minute)
+			t.setResetTime(&t.fiveHour, time.Unix(ts, 0).UTC().Truncate(time.Minute), "5h")
 		}
 	}
 	if v := h.Get("Anthropic-Ratelimit-Unified-7d-Utilization"); v != "" {
@@ -81,7 +81,7 @@ func (t *Tracker) UpdateFromHeaders(h http.Header) {
 	}
 	if v := h.Get("Anthropic-Ratelimit-Unified-7d-Reset"); v != "" {
 		if ts, err := strconv.ParseInt(v, 10, 64); err == nil {
-			t.sevenDay.ResetsAt = time.Unix(ts, 0).UTC().Truncate(time.Minute)
+			t.setResetTime(&t.sevenDay, time.Unix(ts, 0).UTC().Truncate(time.Minute), "7d")
 		}
 	}
 }
@@ -190,24 +190,25 @@ func (t *Tracker) updateWindow(w *Window, rl *RateLimit, name string) {
 	if rl.ResetsAt != nil {
 		parsed, err := time.Parse(time.RFC3339Nano, *rl.ResetsAt)
 		if err != nil {
-			// Fallback to RFC3339 without fractional seconds
 			parsed, err = time.Parse(time.RFC3339, *rl.ResetsAt)
 		}
-		parsed = parsed.UTC().Truncate(time.Minute)
-		if err == nil && parsed != w.ResetsAt && !w.ResetsAt.IsZero() {
-			// Window reset detected — zero token counters
+		if err == nil {
+			t.setResetTime(w, parsed.UTC().Truncate(time.Minute), name)
+		}
+	}
+}
+
+func (t *Tracker) setResetTime(w *Window, newReset time.Time, name string) {
+	if !w.ResetsAt.IsZero() && newReset != w.ResetsAt {
 		log.Info().
 			Str("window", name).
 			Int64("prev_tokens_in", w.TokensIn.Load()).
 			Int64("prev_tokens_out", w.TokensOut.Load()).
 			Time("old_reset", w.ResetsAt).
-				Time("new_reset", parsed).
+			Time("new_reset", newReset).
 			Msg("window reset detected")
 		w.TokensIn.Store(0)
 		w.TokensOut.Store(0)
 	}
-		if err == nil {
-			w.ResetsAt = parsed
-		}
-	}
+	w.ResetsAt = newReset
 }