chore: add sonnet-4-6 mapping, config defaults, and CI improvements

- Add claude-sonnet-4-6 to default Antigravity model mapping
- Add antigravity_extra_retries default value in config
- Add cache-dependency-path to CI setup-go for faster builds
- Simplify vitest config to avoid vite plugin compatibility issues

backend/internal/config/config.go

@@ -1158,6 +1158,7 @@ func setDefaults() {
 	viper.SetDefault("gateway.force_codex_cli", false)
 	viper.SetDefault("gateway.openai_passthrough_allow_timeout_headers", false)
 	viper.SetDefault("gateway.antigravity_fallback_cooldown_minutes", 1)
+	viper.SetDefault("gateway.antigravity_extra_retries", 10)
 	viper.SetDefault("gateway.max_body_size", int64(100*1024*1024))
 	viper.SetDefault("gateway.upstream_response_read_max_bytes", int64(8*1024*1024))
 	viper.SetDefault("gateway.proxy_probe_response_read_max_bytes", int64(1024*1024))

backend/internal/domain/constants.go

@@ -74,6 +74,7 @@ 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/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",