test(contract): 第2刀c Go 契约快照(/v1/me·usage·UsageEntry)+ Go CI job

支柱 2(契约单源)后端守门 + 两端对账:
- httpapi/contract_test.go：冻结 /v1/me 字段面(12 key,与 Dart _frozenMeKeys 对账,
  注明后端多发 uuid/dp_uuid + expire_at 别名)
- usage/contract_test.go：冻结 /v1/usage(UsagePoint)、/v1/usage/devices
  (DeviceUsagePoint + envelope)字段面
- pb/agentv1/contract_test.go：冻结 agent↔控制面 UsageEntry 字段面
- ci.yml：新增 go-server job(golang:1.25 build+test)——此前 server 测试未进 CI
- 本地:5 契约用例 + 全量 go test ./...(25 包)绿

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/usage/contract_test.go

@@ -0,0 +1,70 @@
+package usage
+
+// contract_test.go — HTTP 契约快照(支柱 2:契约单源)。
+//
+// 冻结 /v1/usage(UsagePoint)与 /v1/usage/devices(deviceUsageResponse + DeviceUsagePoint)
+// 的字段面。改字段会让本测试失败,逼同步客户端 model + 文档。
+//
+// ⚠️ 与客户端冻结集对账:client/test/contract/api_contract_test.dart 的
+// _frozenUsagePointKeys / _frozenDeviceUsageKeys。两端必须一致。
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func jsonTagSet(v any) map[string]struct{} {
+	t := reflect.TypeOf(v)
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	set := map[string]struct{}{}
+	for i := 0; i < t.NumField(); i++ {
+		tag := t.Field(i).Tag.Get("json")
+		if tag == "" || tag == "-" {
+			continue
+		}
+		name := strings.SplitN(tag, ",", 2)[0]
+		if name != "" {
+			set[name] = struct{}{}
+		}
+	}
+	return set
+}
+
+func assertFrozenKeys(t *testing.T, name string, got map[string]struct{}, want ...string) {
+	t.Helper()
+	wantSet := make(map[string]struct{}, len(want))
+	for _, k := range want {
+		wantSet[k] = struct{}{}
+	}
+	for k := range got {
+		if _, ok := wantSet[k]; !ok {
+			t.Errorf("%s: 出现未冻结字段 %q —— 改契约需同步另一端(客户端 model)+ 文档", name, k)
+		}
+	}
+	for _, k := range want {
+		if _, ok := got[k]; !ok {
+			t.Errorf("%s: 缺失冻结字段 %q", name, k)
+		}
+	}
+}
+
+func TestContractUsagePoint(t *testing.T) {
+	assertFrozenKeys(t, "/v1/usage UsagePoint", jsonTagSet(UsagePoint{}),
+		"date", "bytes_up", "bytes_down", "minutes_used",
+	)
+}
+
+func TestContractDeviceUsagePoint(t *testing.T) {
+	assertFrozenKeys(t, "/v1/usage/devices DeviceUsagePoint", jsonTagSet(DeviceUsagePoint{}),
+		"uuid", "name", "platform", "bytes_up", "bytes_down", "minutes_used",
+	)
+}
+
+func TestContractDeviceUsageResponse(t *testing.T) {
+	assertFrozenKeys(t, "/v1/usage/devices envelope", jsonTagSet(deviceUsageResponse{}),
+		"devices",
+	)
+}