fix(host-agent): a large update records its package count (#6794)

The root helper counted updated packages from the last 2,000 characters
of the package manager's output. Every manager prints its count (apt's
"N upgraded, N newly installed", dnf's "Upgrade N Packages", zypper's
"N packages to upgrade", pacman's "Packages (N)") before the long log of
what it then does, so a large update pushed the line out of the tail
and the job recorded no count. Docker03's 2026-11-01 job applied 17
updates and reported none; Docker02's single package was counted.

The update steps now read their whole output (run(keep=None)), and only
the failure text and the job's detail are cut short. Agent 1.26.1, so
hosts show the helper as an update to take.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/plugins/host_agent/test_maint_helper.py

@@ -270,7 +270,7 @@ def test_a_zypper_update_that_asks_for_a_reboot_still_worked(maint, monkeypatch)
     monkeypatch.setattr(maint, "_os_release_id", lambda path="/etc/os-release": "sles")
     ran = []
 
-    def fake_run(cmd, deadline=0, env=None, ok_codes=(0,)):
+    def fake_run(cmd, deadline=0, env=None, ok_codes=(0,), keep=0):
         ran.append(cmd[2])
         code = 102 if cmd[2] == "update" else 0
         return code in ok_codes, "3 packages to upgrade.\n"
@@ -282,6 +282,20 @@ def test_a_zypper_update_that_asks_for_a_reboot_still_worked(maint, monkeypatch)
     assert ran == ["refresh", "update"]
 
 
+def test_a_large_update_still_counts_its_packages(maint, monkeypatch):
+    """apt prints its count before the log of unpacking and setting up each
+    package, which runs far past what a detail keeps."""
+    _manager(monkeypatch, maint, ("apt-get",))
+    log = "17 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.\n" + \
+        "Setting up libfoo (1.2-3) ...\n" * 200
+    monkeypatch.setattr(maint, "run", lambda cmd, **kw: (True, log if kw.get("keep", 0) is None
+                                                         else log[-maint.OUTPUT_KEPT_CHARS:]))
+    monkeypatch.setattr(maint, "reboot_required", lambda: False)
+    got = maint.do_update()
+    assert got["ok"] and got["packages"] == 17
+    assert len(got["detail"]) <= 500
+
+
 def test_a_failed_step_says_which(maint, monkeypatch):
     _manager(monkeypatch, maint, ("pacman",))
     monkeypatch.setattr(maint, "run", lambda cmd, **kw: (False, "error: failed to synchronize all databases"))