✨ 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/pkg/antigravity/client_test.go

@@ -0,0 +1,76 @@
+package antigravity
+
+import (
+	"testing"
+)
+
+func TestExtractProjectIDFromOnboardResponse(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		resp map[string]any
+		want string
+	}{
+		{
+			name: "nil response",
+			resp: nil,
+			want: "",
+		},
+		{
+			name: "empty response",
+			resp: map[string]any{},
+			want: "",
+		},
+		{
+			name: "project as string",
+			resp: map[string]any{
+				"cloudaicompanionProject": "my-project-123",
+			},
+			want: "my-project-123",
+		},
+		{
+			name: "project as string with spaces",
+			resp: map[string]any{
+				"cloudaicompanionProject": "  my-project-123  ",
+			},
+			want: "my-project-123",
+		},
+		{
+			name: "project as map with id",
+			resp: map[string]any{
+				"cloudaicompanionProject": map[string]any{
+					"id": "proj-from-map",
+				},
+			},
+			want: "proj-from-map",
+		},
+		{
+			name: "project as map without id",
+			resp: map[string]any{
+				"cloudaicompanionProject": map[string]any{
+					"name": "some-name",
+				},
+			},
+			want: "",
+		},
+		{
+			name: "missing cloudaicompanionProject key",
+			resp: map[string]any{
+				"otherField": "value",
+			},
+			want: "",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := extractProjectIDFromOnboardResponse(tc.resp)
+			if got != tc.want {
+				t.Fatalf("extractProjectIDFromOnboardResponse() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}