A union type should not merge class fields

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### 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_

Reproduisez l’exemple annoté TypeA|TypeB dans l’extension Visual Studio Code, puis suivez la manière dont le language server fusionne les champs des classes d’union. C’est terminé lorsque le discriminateur sélectionne la classe correcte et signale le champ commun non concordant commonField ainsi que les champs spécifiques aux membres présentés dans l’exemple.

Rédigé par le modèle d'indexation à partir du texte de l'issue.