Defining table fields of a class across multiple files breaks auto-complete

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Completion

### Expected Behaviour

If we have a class such as:
```lua
---@class MyClass
MyClass = {
SomeTable = {
A = "Something",
},
}
```
Then another definition of it in another file (an "extension" of the class):
```lua

-- This file extends the MyClass class.

---@class MyClass
local MyClass = MyClass

MyClass.SomeTable.B = "Other thing"
```
Trying to access `MyClass.SomeTable` should show both `A` and `B` as options in auto-complete.

### Actual Behaviour

If the keys of a table within a class are defined across multiple files, auto-complete will not work for them all. If you access these keys however, the hover tooltip will be correct and go-to-definition will work.


This is best understood with an example folder. I've attached it below and its code is also shown in the reproduction steps.

### Reproduction steps

If you have a class definition in one file such as:
`Class_Main.lua`:
```lua

---@class MyClass
MyClass = {
SomeTable = {
A = "Something",
},
}
```
And you try to add keys to `SomeTable` in another file such as:
`Class_Extensions.lua`
```lua

-- This file extends the MyClass class.

---@class MyClass
local MyClass = MyClass

MyClass.SomeTable.B = "Other thing"
```

Then trying to access `Class.SomeTable.` in another file **will not** show `B` as an option in auto-complete. `B` will only show in auto-complete in the file were it was added, despite `Class` there being tagged with `@class` - which I would expect to add `B` to the class definition and not just the local usage of it.

`UserCode.lua`:

As mentioned previously, if you do access `B` by typing it manually, the hover tooltip will be correct and Ctrl+Click to go to definition will work as expected.


### Additional Notes

Example folder replicating the issue is attached. It's the same example as the one shown above.
[LLS_ClassExtensionBug.zip](https://github.com/LuaLS/lua-language-server/files/12551065/LLS_ClassExtensionBug.zip)

### Log File

_No response_

Reproduce the issue using Class_Main.lua, Class_Extensions.lua, and UserCode.lua from the report or attached example in the Visual Studio Code extension. Start by tracing completion for MyClass.SomeTable across the files, comparing it with the already-working hover and go-to-definition behavior. Done means completion lists both A and B when the table is accessed from another file.

Written by the indexing model from the issue text.