Fix bugs in LFS meta garbage collection (#26122) (#26157)

Backport #26122 by @Zettat123

This PR

- Fix #26093. Replace `time.Time` with `timeutil.TimeStamp`
- Fix #26135. Add missing `xorm:"extends"` to `CountLFSMetaObject` for
LFS meta object query
- Add a unit test for LFS meta object garbage collection

Co-authored-by: Zettat123 <zettat123@gmail.com>
(cherry picked from commit a12d036a68)

models/git/lfs.go

@@ -6,7 +6,6 @@ package git
 import (
 	"context"
 	"fmt"
-	"time"
 
 	"code.gitea.io/gitea/models/db"
 	"code.gitea.io/gitea/models/perm"
@@ -370,8 +369,8 @@ func IterateRepositoryIDsWithLFSMetaObjects(ctx context.Context, f func(ctx cont
 
 // IterateLFSMetaObjectsForRepoOptions provides options for IterateLFSMetaObjectsForRepo
 type IterateLFSMetaObjectsForRepoOptions struct {
-	OlderThan                 time.Time
-	UpdatedLessRecentlyThan   time.Time
+	OlderThan                 timeutil.TimeStamp
+	UpdatedLessRecentlyThan   timeutil.TimeStamp
 	OrderByUpdated            bool
 	LoopFunctionAlwaysUpdates bool
 }
@@ -382,8 +381,8 @@ func IterateLFSMetaObjectsForRepo(ctx context.Context, repoID int64, f func(cont
 	batchSize := setting.Database.IterateBufferSize
 	engine := db.GetEngine(ctx)
 	type CountLFSMetaObject struct {
-		Count int64
-		LFSMetaObject
+		Count         int64
+		LFSMetaObject `xorm:"extends"`
 	}
 
 	id := int64(0)