Fix CUDA downloads on unsupported platforms (#770)

* Fix CUDA downloads on unsupported platforms

* fix: align CUDA status nullability

* fix: require CUDA download support flag

backend/tests/test_cuda_download.py

@@ -0,0 +1,32 @@
+import sys as py_sys
+import types
+
+import pytest
+
+from backend.services import cuda
+
+
+def test_cuda_status_reports_unsupported_linux_download(monkeypatch, tmp_path):
+    monkeypatch.setattr(cuda.sys, "platform", "linux")
+    monkeypatch.setattr(cuda, "get_data_dir", lambda: tmp_path)
+
+    status = cuda.get_cuda_status()
+
+    assert status["available"] is False
+    assert status["download_supported"] is False
+    assert status["unsupported_reason"] == cuda.CUDA_DOWNLOAD_UNSUPPORTED_REASON
+
+
+@pytest.mark.asyncio
+async def test_cuda_download_rejects_linux_before_network(monkeypatch, tmp_path):
+    monkeypatch.setattr(cuda.sys, "platform", "linux")
+    monkeypatch.setattr(cuda, "get_data_dir", lambda: tmp_path)
+
+    class UnexpectedClient:
+        def __init__(self, *args, **kwargs):
+            raise AssertionError("unsupported platforms should not start a release download")
+
+    monkeypatch.setitem(py_sys.modules, "httpx", types.SimpleNamespace(AsyncClient=UnexpectedClient))
+
+    with pytest.raises(RuntimeError, match="currently only published for Windows"):
+        await cuda._download_cuda_binary_locked("v0.5.0")