literal strings 'async' is mis-parsed

Nobody has claimed this yet.

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations

### Expected Behaviour

The following should work without errors:
```lua
---@class Unit
---@field mode "async" | "sync"
```

and:
```lua
---@type Unit
local test = {}

test.mode = (hint) --> "async" or "sync"
```

### Actual Behaviour

The follow shows an error
```lua
---@class Unit
---@field mode "async" (error here: expected.) | "sync"
```

and:
```lua
---@type Unit
local test = {}

test.mode = (hint) --> "sync"
```

### Reproduction steps

1. Copy and paste the expected behavior codeblock.

### Additional Notes

Only happens with `async` as far as I can see.

### Log File

_No response_

Start by reproducing the annotation examples in the Visual Studio Code extension and compare how the parser handles the literal string "async" with "sync". Trace the annotation/type-name handling that produces the expected error, then add coverage showing both literal values are accepted without changing the reported type hint.

Written by the indexing model from the issue text.