feat(backend): 添加 Gemini/Google API 基础包

- 新增 pkg/gemini: 模型定义与回退列表
- 新增 pkg/googleapi: Google API 错误状态处理
- 新增 pkg/geminicli/models.go: CLI 模型结构
- 更新 constants.go: AI Studio 相关常量
- 更新 oauth.go: 支持 AI Studio OAuth 流程，凭据通过环境变量配置

backend/internal/pkg/gemini/models.go

@@ -0,0 +1,43 @@
+package gemini
+
+// This package provides minimal fallback model metadata for Gemini native endpoints.
+// It is used when upstream model listing is unavailable (e.g. OAuth token missing AI Studio scopes).
+
+type Model struct {
+	Name                       string   `json:"name"`
+	DisplayName                string   `json:"displayName,omitempty"`
+	Description                string   `json:"description,omitempty"`
+	SupportedGenerationMethods []string `json:"supportedGenerationMethods,omitempty"`
+}
+
+type ModelsListResponse struct {
+	Models []Model `json:"models"`
+}
+
+func DefaultModels() []Model {
+	methods := []string{"generateContent", "streamGenerateContent"}
+	return []Model{
+		{Name: "models/gemini-3-pro-preview", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-2.0-flash", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-2.0-flash-lite", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-1.5-pro", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-1.5-flash", SupportedGenerationMethods: methods},
+		{Name: "models/gemini-1.5-flash-8b", SupportedGenerationMethods: methods},
+	}
+}
+
+func FallbackModelsList() ModelsListResponse {
+	return ModelsListResponse{Models: DefaultModels()}
+}
+
+func FallbackModel(model string) Model {
+	methods := []string{"generateContent", "streamGenerateContent"}
+	if model == "" {
+		return Model{Name: "models/unknown", SupportedGenerationMethods: methods}
+	}
+	if len(model) >= 7 && model[:7] == "models/" {
+		return Model{Name: model, SupportedGenerationMethods: methods}
+	}
+	return Model{Name: "models/" + model, SupportedGenerationMethods: methods}
+}
+