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_

Reproduce the annotated TypeA|TypeB example in the Visual Studio Code extension, then trace how the language server merges union class fields. Done means the discriminator selects the correct class and reports the mismatched commonField and member-specific fields shown in the example.

Written by the indexing model from the issue text.