test(antigravity): add missing unit tests for upstream and custom model_mapping

- Add GetAccessToken upstream branch tests (success/failure/empty/nil)
- Add mapAntigravityModel wildcard-target-equals-request edge case tests
- Add upstream account smart retry test case
- Add GeminiMessagesCompatService custom model_mapping and empty model tests

backend/internal/service/gemini_multiplatform_test.go

@@ -905,6 +905,39 @@ func TestGeminiMessagesCompatService_isModelSupportedByAccount(t *testing.T) {
 			model:    "gpt-4",
 			expected: false,
 		},
+		{
+			name:     "Antigravity平台-空模型允许",
+			account:  &Account{Platform: PlatformAntigravity},
+			model:    "",
+			expected: true,
+		},
+		{
+			name: "Antigravity平台-自定义映射-支持自定义模型",
+			account: &Account{
+				Platform: PlatformAntigravity,
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{
+						"my-custom-model": "upstream-model",
+						"gpt-4o":          "some-model",
+					},
+				},
+			},
+			model:    "my-custom-model",
+			expected: true,
+		},
+		{
+			name: "Antigravity平台-自定义映射-不在映射中的模型不支持",
+			account: &Account{
+				Platform: PlatformAntigravity,
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{
+						"my-custom-model": "upstream-model",
+					},
+				},
+			},
+			model:    "claude-sonnet-4-5",
+			expected: false,
+		},
 		{
 			name:     "Gemini平台-无映射配置-支持所有模型",
 			account:  &Account{Platform: PlatformGemini},