feat(service): 定义 Gemini 服务端口接口

- 定义 OAuth 服务接口
- 定义 Token 缓存服务接口
- 定义 Code Assist 服务接口

backend/internal/service/geminicli_codeassist.go

@@ -0,0 +1,13 @@
+package ports
+
+import (
+	"context"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/geminicli"
+)
+
+// GeminiCliCodeAssistClient calls GeminiCli internal Code Assist endpoints.
+type GeminiCliCodeAssistClient interface {
+	LoadCodeAssist(ctx context.Context, accessToken, proxyURL string, req *geminicli.LoadCodeAssistRequest) (*geminicli.LoadCodeAssistResponse, error)
+	OnboardUser(ctx context.Context, accessToken, proxyURL string, req *geminicli.OnboardUserRequest) (*geminicli.OnboardUserResponse, error)
+}