fix(agent): add fallback for CPU model detection on MIPS architectures (#2138)

gopsutil's cpu.Info() does not parse the 'cpu model' field from
/proc/cpuinfo, which is the only source of CPU model names on MIPS.
Add a fallback that reads /proc/cpuinfo directly and combines
'cpu model' (e.g. 'MIPS 1004Kc V2.15') with 'system type'
(e.g. 'MediaTek MT7621 ver:1 eco:3') for a complete identifier.

The fallback only triggers when gopsutil returns an empty ModelName,
so x86/ARM/other architectures are unaffected.

agent/system_test.go

@@ -1,6 +1,7 @@
 package agent
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/henrygd/beszel/internal/common"
@@ -113,3 +114,81 @@ func TestUpdateSystemDetailsMarksDetailsDirty(t *testing.T) {
 	assert.False(t, agent.detailsDirty)
 	assert.Nil(t, original.Details)
 }
+
+func TestParseCpuModel(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name: "MIPS with both cpu model and system type",
+			input: `system type             : MediaTek MT7621 ver:1 eco:3
+machine                 : ASUS RT-AX53U
+processor               : 0
+cpu model               : MIPS 1004Kc V2.15
+BogoMIPS                : 586.13
+wait instruction        : yes`,
+			expected: "MIPS 1004Kc V2.15 / MediaTek MT7621 ver:1 eco:3",
+		},
+		{
+			name: "MIPS with different SoC",
+			input: `system type             : Atheros AR7161 rev 2
+machine                 : NETGEAR WNDR3700
+processor               : 0
+cpu model               : MIPS 24Kc V7.4
+BogoMIPS                : 452.19`,
+			expected: "MIPS 24Kc V7.4 / Atheros AR7161 rev 2",
+		},
+		{
+			name: "only system type when cpu model missing",
+			input: `system type             : Broadcom BCM47xx
+processor               : 0
+BogoMIPS                : 296.11`,
+			expected: "Broadcom BCM47xx",
+		},
+		{
+			name: "only cpu model when system type missing",
+			input: `processor               : 0
+cpu model               : MIPS 34Kc V2.15
+BogoMIPS                : 300.00`,
+			expected: "MIPS 34Kc V2.15",
+		},
+		{
+			name: "x86 cpuinfo returns empty",
+			input: `processor	: 0
+vendor_id	: GenuineIntel
+cpu family	: 6
+model		: 142
+model name	: Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz
+stepping	: 10`,
+			expected: "",
+		},
+		{
+			name:     "empty input",
+			input:    "",
+			expected: "",
+		},
+		{
+			name: "cpu model with extra whitespace",
+			input: `processor               : 0
+cpu model               :   MIPS 34Kc V2.15
+BogoMIPS                : 300.00`,
+			expected: "MIPS 34Kc V2.15",
+		},
+		{
+			name: "cpu model without value",
+			input: `processor               : 0
+cpu model               :
+BogoMIPS                : 300.00`,
+			expected: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := parseCpuModel(strings.NewReader(tt.input))
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}