Exact Class with `__index` Assignment Result in Loss of All Class Methods

### 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, Hover

### Expected Behaviour

Method `:method` and `:method2` is shown.

### Actual Behaviour

No method is shown.

### Reproduction steps

Test code:
```lua
---@class (exact) Foo
local Foo = {}
Foo.__index = Foo ---@diagnostic disable-line inject-field

function Foo:method()
end

function Foo:method2(value)
end

---@return Foo
local function foo()
end

local b = foo()
```

Variable `b` has type of `Foo`. However typing `b:` won't show `method` and `method2`. The `self` variable inside `Foo:nethod` and `Foo:method2` will also have same issue if used.

Removing the `(exact)` make it work works as expected. Removing the `__index` assignment also make it works as expected. However, silencing the `__index` inject field with `---@field __index Foo` instead of using `---@diagnostic` still result in same issue.

Hovering to `b` also won't show any of the methods. However hovering to `Foo` works.

### Additional Notes

Typing the method name manually (e.g. `:method2`) will show completion normally and will make `b` show `method2` after it's used once. But only `method2` in that specific case. `method` still not be shown.

### Log File

[file_d%3A_Data_Development_love-11.3-win64_luals-report.log](https://github.com/user-attachments/files/17241460/file_d.3A_Data_Development_love-11.3-win64_luals-report.log)

Reproduce the provided Lua snippet and inspect the completion and hover behavior for exact classes with a __index assignment. Use the reported difference between exact and non-exact classes as the starting comparison. Done means Foo methods appear for b and self in completion and hover, without requiring the method name to be typed first.

Written by the indexing model from the issue text.