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/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