DEVX-1: fix: add User-Agent header to _download in install_tools

tests/unit/test_install_tools.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import platform
+import urllib.request
 from pathlib import Path
 from unittest.mock import patch
 
@@ -66,6 +67,10 @@ class TestDownload:
         with patch("urllib.request.urlopen", return_value=_FakeResponse()) as mock_urlopen:
             install_tools._download("https://example.com/file", dest)
         mock_urlopen.assert_called_once()
+        call_args = mock_urlopen.call_args
+        req = call_args.args[0]
+        assert isinstance(req, urllib.request.Request)
+        assert req.get_header("User-agent") == "devx/install-tools"
         assert dest.read_bytes() == b"data"