Assigning an exact class field is bypassed with array notation

Exact classes can still have fields injected using array notation (`myClass['field'] = 1`) without a warning.

```lua
---@class (exact) foo
---@field bar string
local foo = {}

foo.bar = "1" -- no warning (OK)

foo.baz = "2" -- warning (OK)

foo['baz'] = "3" -- no warning (not OK)
```


Start with the Lua reproducer in the issue and trace the diagnostic path for exact-class field assignments. Compare how dot notation and bracket notation are handled for the same field access. Done means the shown `foo['baz']` assignment produces the same warning as `foo.baz`.

Written by the indexing model from the issue text.