feat(antigravity): map all gemini-2.5 to gemini-3 series

Antigravity 上游不再支持 gemini-2.5 系列，统一映射到 gemini-3：
- gemini-2.5-flash → gemini-3-flash
- gemini-2.5-flash-lite → gemini-3-flash
- gemini-2.5-flash-thinking → gemini-3-flash
- gemini-2.5-flash-image → gemini-3-pro-image
- 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_model_mapping_test.go

@@ -134,12 +134,12 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "claude-sonnet-4-5",
 		},
 
-		// 3. Gemini 透传
+		// 3. Gemini 2.5 → 3 映射
 		{
-			name:           "Gemini透传 - gemini-2.5-flash",
+			name:           "Gemini映射 - gemini-2.5-flash → gemini-3-flash",
 			requestedModel: "gemini-2.5-flash",
 			accountMapping: nil,
-			expected:       "gemini-2.5-flash",
+			expected:       "gemini-3-flash",
 		},
 		{
 			name:           "Gemini映射 - gemini-2.5-pro → gemini-3-pro-high",