A union type should not merge class fields

還沒有人認領這個 Issue。

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

MacOS

### What is the issue affecting?

Annotations

### Expected Behaviour

```lua
---@class (exact) TypeA
---@field type 'A'
---@field commonField number
---@field specificFieldA number

---@class (exact) TypeB
---@field type 'B'
---@field commonField string
---@field specificFieldB string

---@alias Type_Either_A_Or_B_But_Not_Both TypeA|TypeB

---@type Type_Either_A_Or_B_But_Not_Both
local testA = {
type = 'A', --- Indicates that this should be of TypeA
commonField = 'test', --- Should be an error. TypeA.commonField should be a number
specificFieldA = 123, --- OK
specificFieldB = 'Hello' --- Should be an error. TypeA.specificFieldB doesn't exist
}

---@type Type_Either_A_Or_B_But_Not_Both
local testB = {
type = 'B', --- Indicates that this should be of TypeB
commonField = 'test', --- OK
specificFieldA = 123, --- Should be an error. TypeB.specificFieldA doesn't exist
specificFieldB = 'Hello' --- OK
}
```

### Actual Behaviour

`TypeA|TypeB` is merged, so none of the mismatched fields/types are reported as errors.

```
local testA: TypeA|TypeB {
commonField: string|number,
specificFieldA: number,
specificFieldB: string,
type: 'A'|'B',
}
```

### Reproduction steps

Paste the code into vscode

### Additional Notes

_No response_

### Log File

_No response_

在 Visual Studio Code 擴充功能中重現加註的 TypeA|TypeB 範例，然後追蹤 language server 如何合併 union class 欄位。完成的條件是 discriminator 選取正確的 class，並回報範例中顯示的不相符 commonField 和成員專屬欄位。

由索引模型根據 Issue 內容生成。