test(hostagent): stop the standby tests reading the runner's own disks

TestCollectDeviceSMARTStandby and its observability twin stub smartctl's
execution but not the device-class probe, so collectDeviceSMART fell through
to the *runner's* real /sys/block/sda/queue/rotational.

smartctlArgs only sends the -n standby guard when the disk is not a confirmed
SSD (#1516), and smartctlArgsUseStandbyExitStatus gates the standby reading of
exit status 3 on that guard having been sent. On a Linux host whose own
/dev/sda is non-rotational the guard is dropped, exit 3 stops meaning standby,
and both tests fail deterministically:

    run smartctl for /dev/sda: exit status 3

They pass on macOS only because linuxNonRotationalBlockDevice returns false off
Linux, and on Linux only where /dev/sda is absent or spinning. That makes the
release-qualification suite unrunnable on an ordinary SSD-backed Linux worker.

Pin the probed disk to rotational through the package's existing stubLinuxSysfs
seam so the guard is always in play. Product behaviour is untouched; this only
stops two unit tests depending on the hardware underneath them.

internal/hostagent/collector_observability_test.go

@@ -118,6 +118,14 @@ func TestCollectDeviceSMART_LogsStructuredContextWhenDeviceInStandby(t *testing.
 		execLookPath = origLook
 	})
 
+	// Same host-sysfs leak as TestCollectDeviceSMARTStandby: the -n standby
+	// guard, and therefore the standby meaning of exit status 3, depends on
+	// the probed disk not being a confirmed SSD. Pin it to rotational instead
+	// of reading the runner's own /sys/block/sda/queue/rotational.
+	stubLinuxSysfs(t, []string{"sda"}, map[string]string{
+		"/sys/block/sda/queue/rotational": "1\n",
+	})
+
 	execLookPath = func(string) (string, error) { return "smartctl", nil }
 	smartRunCommandOutput = func(ctx context.Context, name string, args ...string) ([]byte, error) {
 		return exec.CommandContext(ctx, "sh", "-c", "exit 3").Output()

internal/hostagent/smartctl_coverage_test.go

@@ -584,6 +584,16 @@ func TestCollectDeviceSMARTStandby(t *testing.T) {
 		timeNow = origNow
 	})
 
+	// smartctlArgs only sends the -n standby guard for disks that are not
+	// confirmed SSDs, and exit status 3 only means standby when that guard
+	// was sent. Without a stubbed sysfs this reads the *runner's* real
+	// /sys/block/sda/queue/rotational, so the case under test evaporates on
+	// any Linux host whose own /dev/sda is an SSD. Pin the probed disk to
+	// rotational so the guard is always in play.
+	stubLinuxSysfs(t, []string{"sda"}, map[string]string{
+		"/sys/block/sda/queue/rotational": "1\n",
+	})
+
 	fixed := time.Date(2024, 2, 3, 4, 5, 6, 0, time.UTC)
 	timeNow = func() time.Time { return fixed }
 	execLookPath = func(string) (string, error) { return "smartctl", nil }