fix(install): self-heal a stuck Electron download (salvage of #42894) (#42998)

* fix(install): self-heal a stuck Electron download on the desktop build

The desktop build downloads Electron (~114MB) from GitHub. A corrupt cached
zip, or a blocked/throttled GitHub release host (the repeating "retrying" log),
hard-failed the install — and install.sh had no recovery at all while
install.ps1 / `hermes desktop` only purged the cache.

All three build paths now escalate on a failed `npm run pack`:
GitHub → purge corrupt electron-*.zip + stale *-unpacked and retry → one retry
via a public Electron mirror (npmmirror.com). @electron/get SHASUM-verifies the
download, and a user-pinned ELECTRON_MIRROR is always respected (never
overridden). Adds a bash clear_electron_build_cache()/_desktop_pack() to mirror
the existing PowerShell/Python helpers.

* test(install): cover the Electron mirror fallback

Verify `hermes desktop` falls back to a mirror when the cache purge finds
nothing, and that a user-pinned ELECTRON_MIRROR is respected (no extra attempt,
not overridden).

* docs(desktop): troubleshoot a stuck Electron download

Document the automatic cache-purge + mirror fallback, how to pin your own
ELECTRON_MIRROR, and how to clear a corrupt cached zip by hand.

* docs(install): correct the Electron mirror trust framing

The mirror-fallback comments and the desktop troubleshooting doc implied
`@electron/get`'s SHASUM check makes the npmmirror.com download safe against
tampering. It doesn't: the SHASUMS256.txt is fetched from the same mirror, so
the check guards against a corrupt/partial download, not a compromised mirror.

Reframe all four surfaces (install.sh, install.ps1, `hermes desktop`, and the
docs) to state the trust trade-off honestly — npmmirror.com is the de-facto
Electron community mirror, we only fall back to it after the canonical GitHub
download fails, and a user-pinned ELECTRON_MIRROR is never overridden. No
behavior change.

---------

Co-authored-by: xxxigm <tuancanhnguyen706@gmail.com>

tests/hermes_cli/test_gui_command.py

@@ -498,11 +498,42 @@ def test_gui_retries_pack_once_after_purging_build_cache(tmp_path, monkeypatch):
     assert mock_run.call_args_list[2].args[0] == [str(packaged_exe)]
 
 
-def test_gui_does_not_retry_when_purge_finds_nothing(tmp_path, monkeypatch, capsys):
-    """If the purge clears nothing, there's no point retrying — fail fast."""
+def test_gui_falls_back_to_mirror_when_purge_finds_nothing(tmp_path, monkeypatch, capsys):
+    """Purge clears nothing (not a cache problem) → fall back to an Electron
+    mirror once before failing, so a GitHub-blocked download self-heals."""
     root = _make_desktop_tree(tmp_path)
     monkeypatch.setattr(cli_main, "PROJECT_ROOT", root)
     _make_packaged_executable(root, monkeypatch, platform="linux")
+    monkeypatch.delenv("ELECTRON_MIRROR", raising=False)
+
+    install_ok = subprocess.CompletedProcess(["npm", "ci"], 0)
+    pack_fail = subprocess.CompletedProcess(["npm", "run", "pack"], 1)
+
+    with patch("hermes_cli.main.shutil.which", return_value="/usr/bin/npm"), \
+         patch("hermes_cli.main._run_npm_install_deterministic", return_value=install_ok), \
+         patch("hermes_cli.main._desktop_macos_relaunchable_fixup"), \
+         patch("hermes_cli.main._purge_electron_build_cache", return_value=[]) as mock_purge, \
+         patch("hermes_cli.main.subprocess.run", side_effect=[pack_fail, pack_fail]) as mock_run, \
+         pytest.raises(SystemExit) as exc:
+        cli_main.cmd_gui(_ns())
+
+    assert exc.value.code == 1
+    mock_purge.assert_called_once()
+    # pack(fail) → purge(nothing) → pack via mirror(fail) = 2 subprocess.run calls
+    assert mock_run.call_count == 2
+    # The retry runs the same build but with ELECTRON_MIRROR injected.
+    assert "ELECTRON_MIRROR" not in (mock_run.call_args_list[0].kwargs.get("env") or {})
+    assert mock_run.call_args_list[1].kwargs["env"]["ELECTRON_MIRROR"]
+    assert "Desktop GUI build failed" in capsys.readouterr().out
+
+
+def test_gui_does_not_override_user_electron_mirror(tmp_path, monkeypatch, capsys):
+    """A user-pinned ELECTRON_MIRROR is respected: no extra mirror fallback
+    attempt (and we never swap in our default mirror)."""
+    root = _make_desktop_tree(tmp_path)
+    monkeypatch.setattr(cli_main, "PROJECT_ROOT", root)
+    _make_packaged_executable(root, monkeypatch, platform="linux")
+    monkeypatch.setenv("ELECTRON_MIRROR", "https://mirror.example/electron/")
 
     install_ok = subprocess.CompletedProcess(["npm", "ci"], 0)
     pack_fail = subprocess.CompletedProcess(["npm", "run", "pack"], 1)
@@ -518,6 +549,7 @@ def test_gui_does_not_retry_when_purge_finds_nothing(tmp_path, monkeypatch, caps
     assert exc.value.code == 1
     mock_purge.assert_called_once()
     assert mock_run.call_count == 1
+    assert mock_run.call_args_list[0].kwargs["env"]["ELECTRON_MIRROR"] == "https://mirror.example/electron/"
     assert "Desktop GUI build failed" in capsys.readouterr().out