feat(antigravity): gemini-2.5-flash-image 转发到 gemini-3-pro-image

backend/internal/service/antigravity_gateway_service.go

@@ -62,7 +62,8 @@ var antigravityPrefixMapping = []struct {
 	target string
 }{
 	// 长前缀优先
-	{"gemini-3-pro-image", "gemini-3-pro-image"}, // gemini-3-pro-image-preview 等
+	{"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
 	{"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