fix: stop prepared statement cache cleanup goroutine on Close

The expirable LRU cache backing the prepared statement store starts a
background deleteExpired goroutine when it is created with a non-zero TTL.
Its Close method was commented out, so the goroutine kept running for the
lifetime of the process and leaked every time a PreparedStmtDB was
discarded (for example when a *sql.DB is closed and reopened), causing
steady goroutine and memory growth in long-running services.

Expose LRU.Close, add Close to the stmt_store.Store interface and its
lruStore implementation, and call it from PreparedStmtDB.Close so the
background cleanup goroutine is stopped together with the cached
statements. Close is idempotent and safe for concurrent use.

Refs #7788

tests/lru_test.go

@@ -6,6 +6,7 @@ import (
 	"math"
 	"math/big"
 	"reflect"
+	"runtime"
 	"sync"
 	"testing"
 	"time"
@@ -13,6 +14,49 @@ import (
 	"gorm.io/gorm/internal/lru"
 )
 
+// TestLRU_Close_StopsBackgroundGoroutine verifies that Close terminates the
+// background deleteExpired goroutine started for a cache with a non-zero TTL.
+// Without Close, every such cache leaks one goroutine for the lifetime of the
+// process (see go-gorm/gorm issue #7788).
+func TestLRU_Close_StopsBackgroundGoroutine(t *testing.T) {
+	const n = 100
+
+	runtime.GC()
+	time.Sleep(50 * time.Millisecond)
+	baseline := runtime.NumGoroutine()
+
+	caches := make([]*lru.LRU[string, int], 0, n)
+	for i := 0; i < n; i++ {
+		// A non-zero TTL starts a background deleteExpired goroutine per cache.
+		caches = append(caches, lru.NewLRU[string, int](16, nil, time.Minute))
+	}
+
+	// Sanity check: the background goroutines are running.
+	if got := runtime.NumGoroutine(); got < baseline+n/2 {
+		t.Fatalf("expected background goroutines to start: got %d, baseline %d", got, baseline)
+	}
+
+	for _, c := range caches {
+		c.Close()
+	}
+	caches = nil
+
+	// After Close the background goroutines must terminate and the goroutine
+	// count must return close to the baseline.
+	deadline := time.Now().Add(2 * time.Second)
+	for {
+		runtime.GC()
+		got := runtime.NumGoroutine()
+		if got <= baseline+n/10 {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("background goroutines leaked after Close: got %d, baseline %d", got, baseline)
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+}
+
 func TestLRU_Add_ExistingKey_UpdatesValueAndExpiresAt(t *testing.T) {
 	lru := lru.NewLRU[string, int](10, nil, time.Hour)
 	lru.Add("key1", 1)