feat: 新增 claude-sonnet-4-6 模型支持

- 后端默认映射、身份注入、迁移文件
- 前端模型列表和快捷映射按钮

backend/internal/domain/constants.go

@@ -73,6 +73,7 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"claude-opus-4-6-thinking":   "claude-opus-4-6-thinking", // 官方模型
 	"claude-opus-4-6":            "claude-opus-4-6-thinking", // 简称映射
 	"claude-opus-4-5-thinking":   "claude-opus-4-6-thinking", // 迁移旧模型
+	"claude-sonnet-4-6":          "claude-sonnet-4-6",
 	"claude-sonnet-4-5":          "claude-sonnet-4-5",
 	"claude-sonnet-4-5-thinking": "claude-sonnet-4-5-thinking",
 	// Claude 详细版本 ID 映射

backend/internal/pkg/antigravity/request_transformer.go

@@ -206,6 +206,7 @@ type modelInfo struct {
 var modelInfoMap = map[string]modelInfo{
 	"claude-opus-4-5":   {DisplayName: "Claude Opus 4.5", CanonicalID: "claude-opus-4-5-20250929"},
 	"claude-opus-4-6":   {DisplayName: "Claude Opus 4.6", CanonicalID: "claude-opus-4-6"},
+	"claude-sonnet-4-6": {DisplayName: "Claude Sonnet 4.6", CanonicalID: "claude-sonnet-4-6"},
 	"claude-sonnet-4-5": {DisplayName: "Claude Sonnet 4.5", CanonicalID: "claude-sonnet-4-5-20250929"},
 	"claude-haiku-4-5":  {DisplayName: "Claude Haiku 4.5", CanonicalID: "claude-haiku-4-5-20251001"},
 }

backend/internal/service/antigravity_model_mapping_test.go

@@ -76,6 +76,12 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 		},
 
 		// 3. 默认映射中的透传（映射到自己）
+		{
+			name:           "默认映射透传 - claude-sonnet-4-6",
+			requestedModel: "claude-sonnet-4-6",
+			accountMapping: nil,
+			expected:       "claude-sonnet-4-6",
+		},
 		{
 			name:           "默认映射透传 - claude-sonnet-4-5",
 			requestedModel: "claude-sonnet-4-5",