fix: 修复 Antigravity 账号 intercept_warmup_requests 配置无法保存

提取 applyInterceptWarmup 纯函数统一所有调用点：
- 修复 upstream 创建时遗漏写入 intercept_warmup_requests
- 修复 apikey 编辑时缺少 else 清除逻辑
- 添加前后端单元测试
- 修复 vitest.config.ts mergeConfig 兼容性问题

backend/internal/service/account_intercept_warmup_test.go

@@ -0,0 +1,66 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAccount_IsInterceptWarmupEnabled(t *testing.T) {
+	tests := []struct {
+		name        string
+		credentials map[string]any
+		expected    bool
+	}{
+		{
+			name:        "nil credentials",
+			credentials: nil,
+			expected:    false,
+		},
+		{
+			name:        "empty map",
+			credentials: map[string]any{},
+			expected:    false,
+		},
+		{
+			name:        "field not present",
+			credentials: map[string]any{"access_token": "tok"},
+			expected:    false,
+		},
+		{
+			name:        "field is true",
+			credentials: map[string]any{"intercept_warmup_requests": true},
+			expected:    true,
+		},
+		{
+			name:        "field is false",
+			credentials: map[string]any{"intercept_warmup_requests": false},
+			expected:    false,
+		},
+		{
+			name:        "field is string true",
+			credentials: map[string]any{"intercept_warmup_requests": "true"},
+			expected:    false,
+		},
+		{
+			name:        "field is int 1",
+			credentials: map[string]any{"intercept_warmup_requests": 1},
+			expected:    false,
+		},
+		{
+			name:        "field is nil",
+			credentials: map[string]any{"intercept_warmup_requests": nil},
+			expected:    false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			a := &Account{Credentials: tt.credentials}
+			result := a.IsInterceptWarmupEnabled()
+			require.Equal(t, tt.expected, result)
+		})
+	}
+}