feat(antigravity): map gemini-2.5-pro to gemini-3-pro-high

Add prefix mapping rules for gemini-2.5-pro variants:
- gemini-2.5-pro -> gemini-3-pro-high
- gemini-2.5-pro-preview -> gemini-3-pro-high
- gemini-2.5-pro-exp -> gemini-3-pro-high

backend/internal/service/antigravity_gateway_service.go

@@ -293,6 +293,9 @@ var antigravityPrefixMapping = []struct {
 	target string
 }{
 	// 长前缀优先
+	{"gemini-2.5-pro-preview", "gemini-3-pro-high"},  // gemini-2.5-pro-preview → gemini-3-pro-high
+	{"gemini-2.5-pro-exp", "gemini-3-pro-high"},      // gemini-2.5-pro-exp → gemini-3-pro-high
+	{"gemini-2.5-pro", "gemini-3-pro-high"},          // gemini-2.5-pro → gemini-3-pro-high
 	{"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