fix: never report "nothing pending" when the package index is gone, and load it on request

OpenWrt keeps its package index in RAM. After a reboot `apk version` only
warns that it cannot open its cached index and exits 0, and `opkg
list-upgradable` prints nothing: both read as "no updates". netOrk keeps
"pending since" per package (MVP 5), so the reader now raises instead:

- apk: on apk's own warning ("opening from cache ... No such file"), or a
  failed exit status.
- opkg: when /var/opkg-lists is empty.
- refresh_available_updates(): apk update / opkg update, success from the
  exit status.

tests/unit/test_updates.py

@@ -0,0 +1,113 @@
+"""Pending updates on OpenWrt: never "nothing pending" when the index is gone.
+
+OpenWrt keeps its package index in RAM. After a reboot ``apk version`` only
+warns that it cannot open the cached index and exits 0, and ``opkg
+list-upgradable`` prints nothing: both looked like "no updates". netOrk keeps
+"pending since" per package, so a reader that cannot read raises instead, and
+``refresh_available_updates`` loads the index (netOrk MVP 5).
+
+The apk warning is the real output of an OpenWrt 25.12.5 access point after a
+reboot.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from napalm_openwrt.openwrt import OpenWrtDriver
+
+APK_INDEX_MISSING = (
+    "WARNING: opening from cache https://downloads.openwrt.org/releases/25.12.5/targets/"
+    "ath79/generic/packages/packages.adb: No such file or directory\n"
+    "WARNING: opening from cache https://downloads.openwrt.org/releases/25.12.5/packages/"
+    "mips_24kc/base/packages.adb: No such file or directory\n"
+    "__OWRT_RC=0\n"
+)
+APK_PENDING = (
+    "Installed:                                Available:\n"
+    "dnsmasq-2.90-r4                         < 2.90-r5\n"
+    "hostapd-common-2024.09.15-r2            < 2024.09.15-r3\n"
+    "__OWRT_RC=0\n"
+)
+OPKG_PENDING = "dnsmasq - 2.90-4 - 2.90-5\n"
+
+
+@pytest.fixture
+def driver():
+    with patch("napalm_openwrt.openwrt.ConnectHandler"):
+        drv = OpenWrtDriver(hostname="192.168.1.1", username="root", password="")
+        drv.device = MagicMock()
+        yield drv
+
+
+def _answer(driver, pm: str, reply: str):
+    """Answer the package-manager probe with *pm* and everything else with *reply*."""
+    sent: list = []
+
+    def send(cmd, **kwargs):
+        sent.append(cmd)
+        if cmd.startswith("command -v apk"):
+            return "/usr/bin/apk" if pm == "apk" else "__no_apk__"
+        return reply
+
+    driver._send_command = send
+    return sent
+
+
+class TestAvailableUpdates:
+    def test_apk_without_its_index_raises_instead_of_reporting_nothing(self, driver):
+        _answer(driver, "apk", APK_INDEX_MISSING)
+
+        with pytest.raises(RuntimeError, match="index"):
+            driver.get_available_updates()
+
+    def test_apk_with_its_index_lists_what_is_pending(self, driver):
+        _answer(driver, "apk", APK_PENDING)
+
+        updates = driver.get_available_updates()
+
+        assert [u["name"] for u in updates] == ["dnsmasq", "hostapd-common"]
+        assert updates[0]["current_version"] == "2.90-r4"
+        assert updates[0]["new_version"] == "2.90-r5"
+
+    def test_apk_that_failed_raises(self, driver):
+        _answer(driver, "apk", "ERROR: unable to lock database\n__OWRT_RC=99\n")
+
+        with pytest.raises(RuntimeError):
+            driver.get_available_updates()
+
+    def test_opkg_without_its_lists_raises(self, driver):
+        _answer(driver, "opkg", "__NO_INDEX__\n")
+
+        with pytest.raises(RuntimeError, match="index"):
+            driver.get_available_updates()
+
+    def test_opkg_with_its_lists(self, driver):
+        _answer(driver, "opkg", OPKG_PENDING)
+
+        assert driver.get_available_updates() == [
+            {"name": "dnsmasq", "current_version": "2.90-4", "new_version": "2.90-5"}
+        ]
+
+
+class TestRefresh:
+    def test_apk_loads_its_index(self, driver):
+        sent = _answer(driver, "apk", "OK: 8912 distinct packages available\n__OWRT_RC=0\n")
+
+        assert driver.refresh_available_updates()["success"] is True
+        assert sent[-1].startswith("apk update")
+
+    def test_opkg_loads_its_lists(self, driver):
+        sent = _answer(driver, "opkg", "Updated list of available packages\n__OWRT_RC=0\n")
+
+        assert driver.refresh_available_updates()["success"] is True
+        assert sent[-1].startswith("opkg update")
+
+    def test_a_failed_refresh_says_why(self, driver):
+        _answer(driver, "apk", "ERROR: network unreachable\n__OWRT_RC=1\n")
+
+        result = driver.refresh_available_updates()
+
+        assert result == {"success": False, "output": "ERROR: network unreachable"}