Function with generic variable return type loses typing after first return

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### 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?

Type Checking

### Expected Behaviour

`ret2` in the example should be listed as `string`

### Actual Behaviour

`ret2` is listed as `unknown`

### Reproduction steps

```lua
---@generic T
---@param arg T
---@return T ...
function GenericReturn(arg) end

local ret1, ret2 = GenericReturn("")
```

### Additional Notes

Changing the return type to `T|string` correctly types the variable return but does not inherit the generic's typing.

### Log File

_No response_

No source file or test is named. Begin by reproducing the generic return and vararg example in the issue, then trace the Lua language server's generic return-type inference and type-checking tests. Done means ret2 is inferred as string without changing the annotation, with a regression test covering the example.

Written by the indexing model from the issue text.