[chore][pkg/stanza/fileconsumer] Use caller context in poll checkpoint save (#47333)

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#### Description

Replace `context.Background()` with the available `ctx` so the
checkpoint save participates in graceful shutdown.

Signed-off-by: Paulo Dias <paulodias.gm@gmail.com>

Author: paulojmdias

pkg/stanza/fileconsumer/file.go

@@ -152,7 +152,7 @@ func (m *Manager) poll(ctx context.Context) {
 	if m.persister != nil {
 		metadata := m.tracker.GetMetadata()
 		if metadata != nil {
-			if err := checkpoint.Save(context.Background(), m.persister, metadata); err != nil {
+			if err := checkpoint.Save(ctx, m.persister, metadata); err != nil {
 				m.set.Logger.Error("save offsets", zap.Error(err))
 			}
 		}