[flake8-unused-arguments] Mark **kwargs in TypeVar as used (ARG001) (#22214)

## Summary

Fixes false positive in ARG001 when `**kwargs` is passed to
`typing.TypeVar`

Closes #22178

When `**kwargs` is used in a `typing.TypeVar` call, the checker was not
recognizing it as a usage, leading to false positive "unused function
argument" warnings.

### Root Cause

In the AST, keyword arguments are represented by the `Keyword` struct
with an `arg` field of type `Option<Identifier>`:
- Named keywords like `bound=int` have `arg = Some("bound")`
- Dictionary unpacking like `**kwargs` has `arg = None`

The existing code only handled the `Some(id)` case, never visiting the
expression when `arg` was `None`, so `**kwargs` was never marked as
used.

### Changes

Added an `else` branch to handle `**kwargs` unpacking by calling
`visit_non_type_definition(value)` when `arg` is `None`. This ensures
the `kwargs` variable is properly visited and marked as used by the
semantic model.

## Test Plan

Tested with the following code:

```python
import typing

def f(
    *args: object,
    default: object = None,
    **kwargs: object,
) -> None:
    typing.TypeVar(*args, **kwargs)
```

Before :

`ARG001 Unused function argument: kwargs
`

After : 

`All checks passed!`

Run the example with the following command(from the root of ruff and
please change the path to the module that contains the code example):

`cargo run -p ruff -- check /path/to/file.py --isolated --select=ARG
--no-cache`

crates/ruff_linter/resources/test/fixtures/flake8_unused_arguments/ARG.py

@@ -1,5 +1,5 @@
 from abc import abstractmethod
-from typing import overload, cast
+from typing import overload, cast, TypeVar
 from typing_extensions import override
 
 
@@ -256,3 +256,15 @@ class C:
         """Docstring."""
         msg = t"{x}..."
         raise NotImplementedError(msg)
+    
+
+###
+# Unused arguments with `**kwargs`.
+###
+
+def f(
+    default: object = None,  # noqa: ARG001
+    **kwargs: object,
+) -> None:
+    TypeVar(**kwargs)
+

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -1878,6 +1878,9 @@ impl<'a> Visitor<'a> for Checker<'a> {
                                 } else {
                                     self.visit_non_type_definition(value);
                                 }
+                            } else {
+                                // Ex: typing.TypeVar(**kwargs)
+                                self.visit_non_type_definition(value);
                             }
                         }
                     }