Widening cast has no effect

还没有人认领这个 Issue。

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations, Type Checking, Hover

### Expected Behaviour

Cast unconditionally overwrites the variable's type with the new type.

### Actual Behaviour

Cast is ignored and in some cases causes additional type check errors.

### Reproduction steps

```lua
---@param str string
local function foo(str)
---@cast str +?
if str == '' then str = nil end
end
```
Note how it produces an error on assignment:
```
This variable is defined as type `string`. Cannot convert its type to `nil`.
- `nil` cannot match `string`
- Type `nil` cannot match `string` Lua Diagnostics.(cast-local-type)
```
Note also that the tooltip for `str` in the comparison says correctly that it is a `string?`. But this has no observable effect on the actual type.

Example two (attempted workaround):
```lua
---@param str string
local function foo(str)
---@cast str string?
if str == '' then str = nil end
end
```
Note how the cast causes a type check error now, which makes no sense.

Example three (from the wiki on cast, modified to actually verify that the cast worked):
```lua
---@type integer
local x

---@cast x +boolean
x = false
```
Note how it produces an error, similarly to the first example. You would expect at least the example from the official documentation itself to work, but it doesn't. This leads me to conclude this is a bug.

### Additional Notes

_No response_

### Log File

_No response_

首先运行 issue 中的三个 Lua 复现，并将悬停信息与赋值诊断进行比较。跟踪局部变量的 cast 处理以及后续的类型检查路径，然后添加或更新覆盖范围，使 widening cast 影响赋值检查，同时不引入相互矛盾的错误。

由索引模型根据 Issue 内容生成。