fix: add gemini-3.1-pro-preview to default Antigravity model mapping

Add missing gemini-3.1-pro-preview -> gemini-3.1-pro-high mapping to
DefaultAntigravityModelMapping for consistency with migration 059.

backend/internal/domain/constants.go

@@ -94,8 +94,9 @@ var DefaultAntigravityModelMapping = map[string]string{
 	"gemini-3-pro-low":   "gemini-3.1-pro-low",
 	"gemini-3-pro-image": "gemini-3-pro-image",
 	// Gemini 3.1 透传
-	"gemini-3.1-pro-high": "gemini-3.1-pro-high",
-	"gemini-3.1-pro-low":  "gemini-3.1-pro-low",
+	"gemini-3.1-pro-high":    "gemini-3.1-pro-high",
+	"gemini-3.1-pro-low":     "gemini-3.1-pro-low",
+	"gemini-3.1-pro-preview": "gemini-3.1-pro-high",
 	// Gemini 3 preview 映射
 	"gemini-3-flash-preview":     "gemini-3-flash",
 	"gemini-3-pro-preview":       "gemini-3.1-pro-high",