fix: Cancellation of slowsort

Cancellation signal wasn't propagated from recursive calls of function,
so in most cases cancellation didn't affect slowsort.

The error was detected by <https://github.com/kisielk/errcheck>.

slowsort.go

@@ -46,13 +46,19 @@ func slowsort[S ~[]E, E any](ctx context.Context, x S, i int, j int, cmp func(a,
 		}
 
 		mid := int(math.Floor(float64(i+j) / 2))
-		slowsort(ctx, x, i, mid, cmp)
-		slowsort(ctx, x, mid+1, j, cmp)
+		if err := slowsort(ctx, x, i, mid, cmp); err != nil {
+			return err
+		}
+		if err := slowsort(ctx, x, mid+1, j, cmp); err != nil {
+			return err
+		}
 		if cmp(x[j], x[mid]) == -1 {
 			x[mid], x[j] = x[j], x[mid]
 		}
 
-		slowsort(ctx, x, i, j-1, cmp)
+		if err := slowsort(ctx, x, i, j-1, cmp); err != nil {
+			return err
+		}
 	}
 
 	return nil

slowsort_test.go

@@ -6,6 +6,7 @@ import (
 	"math"
 	"slices"
 	"testing"
+	"time"
 )
 
 func TestSlowsortFloat(t *testing.T) {
@@ -87,3 +88,20 @@ func TestSlowsortString(t *testing.T) {
 		})
 	}
 }
+
+func TestSlowsortCancellation(t *testing.T) {
+	const testCaseSize = 10000
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	defer cancel()
+	tc := make([]int, testCaseSize)
+	want := make([]int, testCaseSize)
+	for i := 0; i < testCaseSize; i++ {
+		tc[i] = testCaseSize - i
+		want[i] = i
+	}
+
+	err := Slowsort(ctx, tc)
+	if err != context.DeadlineExceeded {
+		t.Errorf("Slowsort(ctx, tc) returned unexpected error: %v", err)
+	}
+}