Request: Limit "Fields cannot be injected" to exact classes

Nobody has claimed this yet.

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

NeoVim

### Which OS are you using?

Windows

### What is the issue affecting?

Annotations

### Expected Behaviour

The [docs say](https://luals.github.io/wiki/annotations/#class):

> Marking the class as (exact) means fields cannot be injected after the definition.

So when I use `@class`, I expect to not get any injection warnings unless I use `(exact)`. I see this came up before in #2710, so I'd add: I should be able to get a warning for injections in exact classes while silencing warnings in non-exact classes.

### Actual Behaviour

Any time I add a field outside of the constructor (and sometimes even ones that are defined in my class constructor), I get this warning:

> Fields cannot be injected into the reference of `ClassX` for `field_y`. To do so, use `---@class` for `instance_z`.

The `@class` annotation makes lua too strict. I'm using lua because I _want_ to be able to inject fields.

Using annotations has multiple effects:
1. It improves completion.
1. It improves symbol lookups like goto definition.
1. It does type checking.

But it's not great that adding annotations to get 1+2 results in tons of warnings from 3.

I understand that people may not want to add `(exact)` all over their codebase, but I don't want to add `---@field [any] any` to all of mine.

I'd propose a change to diagnostic flags to allow independently controlling these two cases:
* `inject-field`: suppress inject field diagnostic for non-exact classes.
* `inject-field-exact`: suppress inject field diagnostic for exact classes.

### Reproduction steps

Using neovim and installed luals with Mason. Version: `pkg:github/luals/lua-language-server@3.17.1`

```lua
-- Stripped-down version of https://github.com/rxi/classic/blob/master/classic.lua with an annotation added to be close to real-world class usage.
--
-- classic
--
-- Copyright (c) 2014, rxi
--
-- This module is free software; you can redistribute it and/or modify it under
-- the terms of the MIT license. See LICENSE for details.

--- @class Object
local Object = {}
Object.__index = Object

function Object:new()
end

function Object:extend()
local cls = {}
for k, v in pairs(self) do
if k:find("__") == 1 then
cls[k] = v
end
end
cls.__index = cls
cls.super = self
setmetatable(cls, self)
return cls
end

function Object:__call(...)
local obj = setmetatable({}, self)
obj:new(...)
return obj
end

-- /end classic

--- @class Item : Object
local Item = Object:extend()

function Item:new(id, category)
self.id = id
self.category = category
self.count = 0
end

function Item:Print()
print("Item", self.id, self.category, self.count)
end

--- @param item Item
function FlagDebugItem(item)
item:Print() -- with the annotation, this completes. Yay!
item.is_cheater = true -- Warning :( "Fields cannot be injected into the reference of `Item` for `is_cheater`."
end
```

If I configure luals to add `inject-field` to the `Lua.diagnostics.disable` list, then the warning goes away. But the warning doesn't come back when I make Item an exact class with this annotation: `--- @class (exact) Item : Object`

**Workaround:** Add a fake field of any type:
```lua
--- @class Item : Object
--- @field [any] any
local Item = Object:extend()
```

### Additional Notes

_No response_

### Log File

```
[WARN][2026-02-23 11:41:41] ...lsp/handlers.lua:564 "More than 100000 files have been scanned. The current scanned directory is `D:\\code\\project\\br`. Please see the [FAQ](https://luals.github.io/wiki/faq/#how-can-i-improve-startup-speeds) to see how you can include fewer files. It is also possible that your [configuration is incorrect](https://luals.github.io/wiki/faq#why-is-the-server-scanning-the-wrong-folder)."
[WARN][2026-02-23 14:32:31] .../lua/vim/lsp.lua:52 "method textDocument/typeDefinition is not supported by any of the servers registered for the current buffer"
[START][2026-02-23 15:29:24] LSP logging initiated
[ERROR][2026-02-23 15:29:24] ...m/lsp/client.lua:620 "LSP[lua_ls]" "Cannot find request with id 125 whilst attempting to cancel"
```

Start by running the provided NeoVim reproduction with LuaLS 3.17.1 and compare `@class` with `@class (exact)`. Then trace how the `inject-field` diagnostic is applied; done means non-exact and exact classes can be controlled independently as proposed, while the documented completion and lookup behavior remains available.

Written by the indexing model from the issue text.