fix: 添加 gemini-3-flash 前缀映射支持 gemini-3-flash-preview

backend/internal/service/antigravity_gateway_service.go

@@ -66,6 +66,7 @@ var antigravityPrefixMapping = []struct {
 	// 长前缀优先
 	{"gemini-2.5-flash-image", "gemini-3-pro-image"}, // gemini-2.5-flash-image → 3-pro-image
 	{"gemini-3-pro-image", "gemini-3-pro-image"},     // gemini-3-pro-image-preview 等
+	{"gemini-3-flash", "gemini-3-flash"},             // gemini-3-flash-preview 等 → gemini-3-flash
 	{"claude-3-5-sonnet", "claude-sonnet-4-5"},       // 旧版 claude-3-5-sonnet-xxx
 	{"claude-sonnet-4-5", "claude-sonnet-4-5"},       // claude-sonnet-4-5-xxx
 	{"claude-haiku-4-5", "claude-sonnet-4-5"},        // claude-haiku-4-5-xxx → sonnet