`F821` does not understand the intention of the following code

### Summary

```python
TYPE_CHECKING = False
if TYPE_CHECKING:
import typing
else:
typing = None
x: typing.Literal["a"] # produces F821: "a" is not defined
```

### Version

ruff 0.16.7 (b5dba861c 2026-09-10)

Start by reproducing the reported snippet with Ruff 0.16.7 and inspect the F821 diagnostic path for Python annotations and TYPE_CHECKING branches. Identify the existing tests covering undefined names or annotations, then add coverage for this example; done means the snippet no longer produces a spurious F821 for the string literal.

Written by the indexing model from the issue text.