fix(#76): prevent false-fresh aggregate responses

internal/api/pacing_test.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"context"
 	"encoding/json"
 	"io"
 	"net/http"
@@ -49,6 +50,28 @@ func TestRequestPacerNilSafe(t *testing.T) {
 	p.wait() // must not panic
 }
 
+func TestRequestPacerCancellationReturnsUnusedReservation(t *testing.T) {
+	p := &requestPacer{interval: 100 * time.Millisecond}
+	if err := p.waitContext(context.Background()); err != nil {
+		t.Fatal(err)
+	}
+	p.mu.Lock()
+	wantNext := p.next
+	p.mu.Unlock()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Millisecond)
+	defer cancel()
+	if err := p.waitContext(ctx); err == nil {
+		t.Fatal("expected paced wait cancellation")
+	}
+	p.mu.Lock()
+	gotNext := p.next
+	p.mu.Unlock()
+	if !gotNext.Equal(wantNext) {
+		t.Fatalf("cancelled reservation left pacing debt: next %v, want %v", gotNext, wantNext)
+	}
+}
+
 func TestGetRetriesOn429(t *testing.T) {
 	var calls atomic.Int32
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {