Catch the 'Nothing(SVC)' placeholder serial in both fallback sites (issue #83)

The ARTIK051_DONGLE_REF firmware family reports the literal string
"Nothing(SVC)" as serialNum on every unit -- non-empty, so the existing
`if not serial` checks in config_flow.py's _probe_and_validate and
coordinator.py's _run_discovery don't catch it. Two such appliances on
one install (a fridge and a freezer, each its own dongle) then collide:
config_flow gives both the same unique_id and rejects the second as
"already configured" (bug 2), and even once that's worked around,
device_serial feeds every entity's unique_id too, so the second
appliance's entities get silently dropped with "does not generate
unique IDs" log lines (bug 4).

Add _is_placeholder_serial (duplicated in both modules rather than
imported, to avoid pulling config_flow into the runtime coordinator's
import graph or vice versa for a two-line check) and treat it the same
as an empty serial: fall back to host/port.

Type detection (bug 1) and the door sensor's field-name gap (bug 3),
also reported in this issue, are already fixed via the
claude/device-support-issue-77-freezer branch, which hit the same
ARTIK051_DONGLE_REF family from a different report -- not duplicated
here.

tests/localthings/test_config_flow.py

@@ -508,3 +508,19 @@ async def test_options_flow_finish_preserves_existing_options(
 
     assert result['type'] == FlowResultType.CREATE_ENTRY
     assert entry.options[CONF_BYPASS_REMOTE_CONTROL] is True
+
+
+def test_is_placeholder_serial_catches_nothing_svc():
+    """Issue #83: the ARTIK051_DONGLE_REF firmware family reports the
+    literal string 'Nothing(SVC)' as serialNum on every unit -- non-empty,
+    so it must be caught by name, not by the plain `if not serial` check."""
+    from custom_components.localthings.config_flow import _is_placeholder_serial
+    assert _is_placeholder_serial('Nothing(SVC)') is True
+    assert _is_placeholder_serial('nothing(svc)') is True
+    assert _is_placeholder_serial('  Nothing(SVC)  ') is True
+
+
+def test_is_placeholder_serial_accepts_real_serials():
+    from custom_components.localthings.config_flow import _is_placeholder_serial
+    assert _is_placeholder_serial('0A1B2C3D4E5F') is False
+    assert _is_placeholder_serial('') is False

tests/localthings/test_coordinator.py

@@ -136,6 +136,29 @@ def test_run_discovery_detects_washer_via_model_fallback(
     assert coordinator.device_type_name == 'washer'
 
 
+def test_run_discovery_falls_back_to_host_for_placeholder_serial(
+    hass: HomeAssistant, mock_entry
+) -> None:
+    """Issue #83: the ARTIK051_DONGLE_REF firmware family reports the
+    literal string 'Nothing(SVC)' as serialNum on every unit. Left as-is,
+    two such units get the same device_serial (which feeds both the HA
+    device-registry identifier and every entity's unique_id), so the
+    second one's entities silently collide and get dropped. It must be
+    treated the same as an empty serial and fall back to the host."""
+    resources = {
+        '/information/vs/0': {
+            'x.com.samsung.da.modelNum':
+                'ARTIK051_DONGLE_REF|00127641|00080020001430300100000000000000',
+            'x.com.samsung.da.description': 'ARTIK_REF_17K',
+            'x.com.samsung.da.serialNum': 'Nothing(SVC)',
+        },
+        '/otninformation/vs/0': {},
+    }
+    coordinator = LocalThingsCoordinator(hass, mock_entry)
+    coordinator._run_discovery(resources)
+    assert coordinator.device_serial == mock_entry.data[CONF_HOST]
+
+
 def test_run_discovery_detects_cooktop_via_resource_signature(
     hass: HomeAssistant, mock_entry
 ) -> None: