RET503 false positive on exhaustive type check

### Summary

From the same codebase as https://github.com/astral-sh/ruff/issues/5474#issuecomment-2968706593 , RET503 still reports an error after an exhaustive type check. If the fix is applied, type checkers will warn/error about unreachable code.
[playground link](https://play.ruff.rs/98b1a534-fca2-4f66-859c-6be9a207f73f)

```py
import tarfile
import zipfile

type ArchiveKind = tarfile.TarFile | zipfile.ZipFile

def get_first_archive_member(archive: ArchiveKind) -> str:
if isinstance(archive, tarfile.TarFile):
return archive.getnames()[0]
elif isinstance(archive, zipfile.ZipFile):
return archive.namelist()[0]
# RET503 error
```

### Version

ruff 0.11.13 (5faf72a4d 2025-06-05) + playground

Start by reproducing the RET503 report with the linked Ruff playground example and inspect the RET503 implementation and its existing tests. The fix is done when the exhaustive TarFile/ZipFile check no longer produces a missing-return diagnostic without causing an unreachable-code warning from type checkers.

Written by the indexing model from the issue text.