assign-type-mismatch not reported for global values

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

NeoVim

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations, Type Checking, Diagnostics/Syntax Checking

### Expected Behaviour

With a global table that has type-annotated fields, reassigning a field to an incorrectly-typed value should report `[assign-type-mismatch]`

### Actual Behaviour

No diagnostic is reported, even though autocompletion indicates that the type is understood by the server. Further, assigning a local variable to the global value and doing the same thing _does_ report the diagnostic correctly.

### Reproduction steps

```lua
---@class test
_G.test = {
---@type boolean
foo = true,
}
test.foo = 'bar' -- no error
_G.test.foo = 'bar' -- no error
local test2 = test
test2.foo = 'bar' -- [assign-type-mismatch]
```

### Additional Notes

_No response_

### Log File

_No response_

Start by running the supplied Lua reproduction and compare diagnostics for test.foo, _G.test.foo, and test2.foo. Trace the type-checking and diagnostics path for global values; done means the two global assignments report [assign-type-mismatch] consistently with the local alias case.

Written by the indexing model from the issue text.