feat(认证): 启用 OpenAI OAuth HTTP/2 并修复清理任务 lint

为共享 req 客户端增加 HTTP/2 选项与缓存隔离
OpenAI OAuth 超时提升到 120s，并按协议控制强制
新增客户端池与 OAuth 客户端单测覆盖
修复 usage cleanup 相关 errcheck/ineffassign/staticcheck 并统一格式

测试: make test

backend/internal/repository/req_client_pool_test.go

@@ -0,0 +1,102 @@
+package repository
+
+import (
+	"reflect"
+	"sync"
+	"testing"
+	"time"
+	"unsafe"
+
+	"github.com/imroc/req/v3"
+	"github.com/stretchr/testify/require"
+)
+
+func forceHTTPVersion(t *testing.T, client *req.Client) string {
+	t.Helper()
+	transport := client.GetTransport()
+	field := reflect.ValueOf(transport).Elem().FieldByName("forceHttpVersion")
+	require.True(t, field.IsValid(), "forceHttpVersion field not found")
+	require.True(t, field.CanAddr(), "forceHttpVersion field not addressable")
+	return reflect.NewAt(field.Type(), unsafe.Pointer(field.UnsafeAddr())).Elem().String()
+}
+
+func TestGetSharedReqClient_ForceHTTP2SeparatesCache(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	base := reqClientOptions{
+		ProxyURL: "http://proxy.local:8080",
+		Timeout:  time.Second,
+	}
+	clientDefault := getSharedReqClient(base)
+
+	force := base
+	force.ForceHTTP2 = true
+	clientForce := getSharedReqClient(force)
+
+	require.NotSame(t, clientDefault, clientForce)
+	require.NotEqual(t, buildReqClientKey(base), buildReqClientKey(force))
+}
+
+func TestGetSharedReqClient_ReuseCachedClient(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	opts := reqClientOptions{
+		ProxyURL: "http://proxy.local:8080",
+		Timeout:  2 * time.Second,
+	}
+	first := getSharedReqClient(opts)
+	second := getSharedReqClient(opts)
+	require.Same(t, first, second)
+}
+
+func TestGetSharedReqClient_IgnoresNonClientCache(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	opts := reqClientOptions{
+		ProxyURL: " http://proxy.local:8080 ",
+		Timeout:  3 * time.Second,
+	}
+	key := buildReqClientKey(opts)
+	sharedReqClients.Store(key, "invalid")
+
+	client := getSharedReqClient(opts)
+
+	require.NotNil(t, client)
+	loaded, ok := sharedReqClients.Load(key)
+	require.True(t, ok)
+	require.IsType(t, "invalid", loaded)
+}
+
+func TestGetSharedReqClient_ImpersonateAndProxy(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	opts := reqClientOptions{
+		ProxyURL:    "  http://proxy.local:8080  ",
+		Timeout:     4 * time.Second,
+		Impersonate: true,
+	}
+	client := getSharedReqClient(opts)
+
+	require.NotNil(t, client)
+	require.Equal(t, "http://proxy.local:8080|4s|true|false", buildReqClientKey(opts))
+}
+
+func TestCreateOpenAIReqClient_ForceHTTP2Enabled(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	client := createOpenAIReqClient("https://auth.openai.com/oauth/token", "http://proxy.local:8080")
+	require.Equal(t, "2", forceHTTPVersion(t, client))
+}
+
+func TestCreateOpenAIReqClient_ForceHTTP2DisabledForHTTP(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	client := createOpenAIReqClient("http://localhost/oauth/token", "http://proxy.local:8080")
+	require.Equal(t, "", forceHTTPVersion(t, client))
+}
+
+func TestCreateOpenAIReqClient_Timeout120Seconds(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	client := createOpenAIReqClient("https://auth.openai.com/oauth/token", "http://proxy.local:8080")
+	require.Equal(t, 120*time.Second, client.GetClient().Timeout)
+}
+
+func TestCreateGeminiReqClient_ForceHTTP2Disabled(t *testing.T) {
+	sharedReqClients = sync.Map{}
+	client := createGeminiReqClient("http://proxy.local:8080")
+	require.Equal(t, "", forceHTTPVersion(t, client))
+}