test(13-01): add failing tests for InterfaceInfo collector

- InterfaceInfo struct field compilation test
- MAC address lowercasing test
- Running bool parsing test

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poller/internal/device/interfaces_test.go

@@ -0,0 +1,72 @@
+package device
+
+import (
+	"testing"
+)
+
+func TestInterfaceInfoFields(t *testing.T) {
+	// Verify struct compiles with expected fields and JSON tags.
+	info := InterfaceInfo{
+		Name:       "ether1",
+		MacAddress: "aa:bb:cc:dd:ee:ff",
+		Type:       "ether",
+		Running:    true,
+	}
+
+	if info.Name != "ether1" {
+		t.Errorf("Name = %q, want %q", info.Name, "ether1")
+	}
+	if info.MacAddress != "aa:bb:cc:dd:ee:ff" {
+		t.Errorf("MacAddress = %q, want %q", info.MacAddress, "aa:bb:cc:dd:ee:ff")
+	}
+	if info.Type != "ether" {
+		t.Errorf("Type = %q, want %q", info.Type, "ether")
+	}
+	if !info.Running {
+		t.Error("Running = false, want true")
+	}
+}
+
+func TestInterfaceMACLowercasing(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{name: "already lowercase", input: "aa:bb:cc:dd:ee:ff", want: "aa:bb:cc:dd:ee:ff"},
+		{name: "uppercase", input: "AA:BB:CC:DD:EE:FF", want: "aa:bb:cc:dd:ee:ff"},
+		{name: "mixed case", input: "Aa:Bb:Cc:Dd:Ee:Ff", want: "aa:bb:cc:dd:ee:ff"},
+		{name: "empty", input: "", want: ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := normalizeMACAddress(tt.input)
+			if got != tt.want {
+				t.Errorf("normalizeMACAddress(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestInterfaceRunningParsing(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  bool
+	}{
+		{name: "true string", input: "true", want: true},
+		{name: "false string", input: "false", want: false},
+		{name: "empty string", input: "", want: false},
+		{name: "yes string", input: "yes", want: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := parseRunning(tt.input)
+			if got != tt.want {
+				t.Errorf("parseRunning(%q) = %v, want %v", tt.input, got, tt.want)
+			}
+		})
+	}
+}