fix: add tenacity retry to install_tools._download for transient network failures

The build-images CI job failed because GitHub releases dropped
connections ("Remote end closed connection without response") for
actionlint, vale, and hadolint simultaneously. The previous code only
fell through to the next fallback URL — there was no retry on the same
URL for transient connection resets.

Now uses tenacity.Retrying with exponential backoff (2-10s, 3 attempts)
on URLError/OSError/ConnectionError, matching the pattern already used
in devx.utils.network. The _sleep kwarg allows tests to skip real
sleeping.

3 new tests cover: retry-then-success, exhaustion-after-max-retries,
and no-retry-on-non-transient-errors.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_install_tools.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import platform
+import urllib.error
 from pathlib import Path
 from unittest.mock import patch
 
@@ -68,6 +69,67 @@ class TestDownload:
         mock_urlopen.assert_called_once()
         assert dest.read_bytes() == b"data"
 
+    def test_download_retries_on_transient_error(self, tmp_path: Path) -> None:
+        """Download retries on URLError then succeeds."""
+        dest = tmp_path / "file.bin"
+        call_count = [0]
+
+        class _FakeResponse:
+            def __init__(self) -> None:
+                self._sent = False
+
+            def __enter__(self) -> _FakeResponse:
+                return self
+
+            def __exit__(self, *args: object) -> None:
+                pass
+
+            def read(self, n: int = -1) -> bytes:
+                if self._sent:
+                    return b""
+                self._sent = True
+                return b"data"
+
+        def _flaky_urlopen(url: str, timeout: int = 60):
+            call_count[0] += 1
+            if call_count[0] < 2:
+                raise urllib.error.URLError("Remote end closed connection")
+            return _FakeResponse()
+
+        with patch("urllib.request.urlopen", side_effect=_flaky_urlopen):
+            install_tools._download("https://example.com/file", dest, _sleep=lambda _: None)
+        assert call_count[0] == 2
+        assert dest.read_bytes() == b"data"
+
+    def test_download_fails_after_max_retries(self, tmp_path: Path) -> None:
+        """Download raises after MAX_DOWNLOAD_RETRIES attempts."""
+        dest = tmp_path / "file.bin"
+        call_count = [0]
+
+        def _always_fail(url: str, timeout: int = 60):
+            call_count[0] += 1
+            raise urllib.error.URLError("Remote end closed connection")
+
+        with patch("urllib.request.urlopen", side_effect=_always_fail):
+            with pytest.raises(urllib.error.URLError):
+                install_tools._download("https://example.com/file", dest, _sleep=lambda _: None)
+        assert call_count[0] == install_tools.MAX_DOWNLOAD_RETRIES
+        assert not dest.exists()
+
+    def test_download_no_retry_on_non_transient_error(self, tmp_path: Path) -> None:
+        """Download does not retry on non-network errors (e.g. ValueError)."""
+        dest = tmp_path / "file.bin"
+        call_count = [0]
+
+        def _fail_with_value_error(url: str, timeout: int = 60):
+            call_count[0] += 1
+            raise ValueError("not a network error")
+
+        with patch("urllib.request.urlopen", side_effect=_fail_with_value_error):
+            with pytest.raises(ValueError):
+                install_tools._download("https://example.com/file", dest, _sleep=lambda _: None)
+        assert call_count[0] == 1
+
 
 class TestDownloadBinary:
     def test_download(self, tmp_path: Path) -> None: