string|nil passed to string parameter not flagged as error with strict settings

Nobody has claimed this yet.

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

Command Line

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking

### Expected Behaviour

With the following config (see below), I expect Lua Language Server to report a type mismatch when passing a `string|nil` value to a function that expects a non-optional string parameter.

Specifically, this line should raise a diagnostic:

```
test_func(params.ip) -- where params.ip is string|nil, but test_func expects string
```

### Actual Behaviour

No error is reported by LLS, neither in VS Code nor when using the CLI with `--check`.

### Reproduction steps

1. Save this code in a `.lua` file:

```
---@class TestParams
---@field ip string | nil

---@param ip string
---@return string
local function test_func(ip)
return ip
end

---@type TestParams
local params = { ip = nil }

test_func(params.ip)

```

2. Use the following `lua-language-server config` (`.luarc.json` or through your client):

```
{
"runtime": {
"version": "Lua 5.1"
},
"diagnostics": {
"enable": true,
"disable": [],
"groupFileStatus": {
"ambiguity": "Any",
"await": "Any",
"codestyle": "Any",
"duplicate": "Any",
"global": "Any",
"luadoc": "Any",
"redefined": "Any",
"strict": "Any",
"strong": "Any",
"type-check": "Any",
"unbalanced": "Any",
"unused": "Any"
},
"groupSeverity": {
"ambiguity": "Error",
"await": "Error",
"codestyle": "Error",
"duplicate": "Error",
"global": "Error",
"luadoc": "Error",
"redefined": "Error",
"strict": "Error",
"strong": "Error",
"type-check": "Error",
"unbalanced": "Error",
"unused": "Error"
}
},
"type": {
"weakNilCheck": false
}
}
```

3. Run the CLI:

```
lua-language-server-3.15.0-linux-x64/bin/lua-language-server --check your_file.lua
```

Or use VS Code extension.

### Additional Notes

_No response_

### Log File

```
Diagnosis completed, no problems found
```

Start with the supplied Lua reproducer and `.luarc.json`, then run the documented `lua-language-server --check` command to confirm the missing diagnostic. Trace the type-checking path for function arguments and add regression coverage; done means the `string|nil` argument is reported as incompatible with the required `string` parameter.

Written by the indexing model from the issue text.