fix(tikv): improve context propagation and refactor batch delete logic (#7558)

* fix(tikv): improve context propagation and refactor batch delete logic

Address review comments from PR #7557:

1. Replace context.TODO() with ctx in txn.Get calls
   - Fixes timeout/cancellation propagation in FindEntry
   - Fixes timeout/cancellation propagation in KvGet

2. Refactor DeleteFolderChildren to use flush helper
   - Eliminates code duplication
   - Cleaner and more maintainable

These changes ensure proper context propagation throughout all
TiKV operations and improve code maintainability.

* error formatting

weed/filer/tikv/tikv_store_kv.go

@@ -27,7 +27,7 @@ func (store *TikvStore) KvGet(ctx context.Context, key []byte) ([]byte, error) {
 	}
 	var data []byte = nil
 	err = tw.RunInTxn(ctx, func(txn *txnkv.KVTxn) error {
-		val, err := txn.Get(context.TODO(), key)
+		val, err := txn.Get(ctx, key)
 		if err == nil {
 			data = val
 		}