Using a union of string literals as a table key doesn't force use of the keys

Nobody has claimed this yet.

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

NeoVim

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking

### Expected Behaviour

```lua
---@alias SomeUnion
--- | 'a'
--- | 'b'

---@type table
local a
```

should work roughly like

```lua
---@class (exact) SomeUnion
---@field a number
---@field b number
local a
```

### Actual Behaviour


### Reproduction steps

```lua
---@alias SomeUnion
--- | 'a'
--- | 'b'

---@type table
local a = {
c = "should be invalid"
}
```

### Additional Notes

_No response_

### Log File

_No response_

Start by reproducing the reported type-checking example in the issue and trace the language server's handling of table keys whose type is a string-literal union. Done means the `c` key in the reproduction is reported invalid while the `a` and `b` keys remain valid.

Written by the indexing model from the issue text.