#975 setup.get_prebuilt_hip() zip-unpack branch returns None on Linux (tools/test_setup_amd.py)

本文

## Summary

On Linux, `setup.get_prebuilt_hip()` returns `None` from its prebuilt-zip unpack branch (no `return` statement), so `tools/test_setup_amd.py::test_prebuilt_hip_zip` fails. On Windows the same call path crashed differently for us (unguarded `sys.exit(1)` in `setup.build_engine_hip` asking for "a C++ compiler and git" — the HIP-compile fallback guard assumes Linux), so the Linux unpack branch is the cleaner repro.

## Environment

- Ubuntu 26.04 LTS (WSL2), x86_64, Python 3.14.4, fresh clone @ `6f32ec0`

## Reproduce

```sh
python3 -m unittest tools/test_setup_amd.py -v
```

```
FAIL: test_prebuilt_hip_zip (__main__.WindowsDetection.test_prebuilt_hip_zip)
AssertionError: None != PosixPath('/tmp/tmpqc1nibf1/engine')
```

`Ran 28 tests ... FAILED (failures=1)` — only this one fails.

## Suggested fix

Make the zip-unpack branch return the unpack destination (the test expects the `engine` dir under the temp dir). While in there, the Windows-side `build_engine_hip` compile-fallback path could also benefit from a guard that reports the missing build tools instead of a bare `sys.exit(1)` (that made the Windows run emit two hard `ERROR`s in the HipVision tests rather than clean failures).