unistore: unrepeatable read

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```go
func TestMaxPositionRepeatableRead(t *testing.T) {
store := testkit.CreateMockStore(t)

tk1 := testkit.NewTestKit(t, store)
tk2 := testkit.NewTestKit(t, store)
tk1.MustExec("use test")
tk2.MustExec("use test")

// Create and initialize the test table
tk1.MustExec("drop table if exists sample_table")
tk1.MustExec(`CREATE TABLE sample_table (
id INT,
position INT,
value VARCHAR(255),
PRIMARY KEY (id, position)
)`)

tk1.MustExec(`INSERT INTO sample_table (id, position, value) VALUES
(1, 1, 'First value'),
(1, 2, 'Second value'),
(2, 1, 'Another value'),
(2, 2, 'Yet another value'),
(3, 1, 'Different id value')`)

// Start transaction in tk2 first
tk2.MustExec("begin")

// Check initial max position
rows := tk2.MustQuery("select max(position) from sample_table where id = 1").Rows()
require.Equal(t, "2", rows[0][0])

// Insert new row in tk1
tk1.MustExec("begin")
tk1.MustExec("insert into sample_table (id, position, value) values (1, 3, 'test')")
tk1.MustExec("commit")

// Check max position again in tk2, should still be 2 due to repeatable read
rows = tk2.MustQuery("select max(position) from sample_table where id = 1").Rows()
require.Equal(t, "2", rows[0][0], "Transaction isolation (repeatable read) violated: saw result from concurrent transaction")

tk2.MustExec("commit")
}
```

### 2. What did you expect to see? (Required)

The last query returns 2.

### 3. What did you see instead (Required)

The last query may return 3.

### 4. What is your TiDB version? (Required)

v6.5, v8.4, and possibly every version in between