[BUGFIX] Ingester: switch test atomic helpers to go.uber.org/atomic

Cortex's faillint lint rule forbids direct use of "sync/atomic" and
requires "go.uber.org/atomic" instead. Replace atomic.Bool and
atomic.Value (storing string) with go.uber.org/atomic.Bool and
go.uber.org/atomic.String in the regression test added for #7531.

No production-code changes; behaviour is identical.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Signed-off-by: Sandy Chen <Yuxuan.Chen@morganstanley.com>

Author: sandy2008

pkg/ingester/active_queried_series_test.go

@@ -4,14 +4,14 @@ import (
 	"context"
 	"fmt"
 	"sync"
-	"sync/atomic"
 	"testing"
 	"time"
 
 	"github.com/go-kit/log"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
 
 	"github.com/cortexproject/cortex/pkg/util/services"
 )
@@ -488,7 +488,7 @@ func TestActiveQueriedSeriesService_NoSendOnClosedChannelOnShutdown(t *testing.T
 
 	var (
 		panicked  atomic.Bool
-		panicMsg  atomic.Value // string
+		panicMsg  atomic.String
 		wg        sync.WaitGroup
 		producing sync.WaitGroup
 		gate      = make(chan struct{}) // released after Stop() returns