✨ feat(antigravity): 添加 onboardUser 支持并修复 project_id 补齐逻辑

- 新增 OnboardUser API 客户端方法，支持账号 onboarding 获取 project_id
- loadProjectIDWithRetry 增加 onboard 回退：LoadCodeAssist 未返回 project_id 时自动触发 onboarding
- GetAccessToken 中 project_id 补齐改用轻量 FillProjectID 替代全量 RefreshAccountToken
- 补齐逻辑增加 5 分钟冷却机制，防止频繁重试
- OnboardUser 轮询等待改为 context 感知，支持提前取消
- 提取 mergeCredentials 辅助方法消除重复代码
- 新增 extractProjectIDFromOnboardResponse 和 resolveDefaultTierID 单元测试

backend/internal/service/antigravity_oauth_service_test.go

@@ -0,0 +1,82 @@
+package service
+
+import (
+	"testing"
+)
+
+func TestResolveDefaultTierID(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		loadRaw map[string]any
+		want    string
+	}{
+		{
+			name:    "nil loadRaw",
+			loadRaw: nil,
+			want:    "",
+		},
+		{
+			name: "missing allowedTiers",
+			loadRaw: map[string]any{
+				"paidTier": map[string]any{"id": "g1-pro-tier"},
+			},
+			want: "",
+		},
+		{
+			name:    "empty allowedTiers",
+			loadRaw: map[string]any{"allowedTiers": []any{}},
+			want:    "",
+		},
+		{
+			name: "tier missing id field",
+			loadRaw: map[string]any{
+				"allowedTiers": []any{
+					map[string]any{"isDefault": true},
+				},
+			},
+			want: "",
+		},
+		{
+			name: "allowedTiers but no default",
+			loadRaw: map[string]any{
+				"allowedTiers": []any{
+					map[string]any{"id": "free-tier", "isDefault": false},
+					map[string]any{"id": "standard-tier", "isDefault": false},
+				},
+			},
+			want: "",
+		},
+		{
+			name: "default tier found",
+			loadRaw: map[string]any{
+				"allowedTiers": []any{
+					map[string]any{"id": "free-tier", "isDefault": true},
+					map[string]any{"id": "standard-tier", "isDefault": false},
+				},
+			},
+			want: "free-tier",
+		},
+		{
+			name: "default tier id with spaces",
+			loadRaw: map[string]any{
+				"allowedTiers": []any{
+					map[string]any{"id": "  standard-tier  ", "isDefault": true},
+				},
+			},
+			want: "standard-tier",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := resolveDefaultTierID(tc.loadRaw)
+			if got != tc.want {
+				t.Fatalf("resolveDefaultTierID() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}