fix(gemini): enable model_mapping filtering for Gemini API Key accounts

Remove the special case that bypassed model-supported checks for Gemini
API Key accounts, allowing model_mapping to filter requests properly.
Add tests for multiplatform model filtering behavior.

backend/internal/service/gateway_multiplatform_test.go

@@ -895,6 +895,55 @@ func TestGatewayService_SelectAccountForModelWithPlatform_GeminiPreferOAuth(t *t
 	require.Equal(t, int64(2), acc.ID)
 }
 
+func TestGatewayService_SelectAccountForModelWithPlatform_GeminiAPIKeyModelMappingFilter(t *testing.T) {
+	ctx := context.Background()
+
+	repo := &mockAccountRepoForPlatform{
+		accounts: []Account{
+			{
+				ID:          1,
+				Platform:    PlatformGemini,
+				Type:        AccountTypeAPIKey,
+				Priority:    1,
+				Status:      StatusActive,
+				Schedulable: true,
+				Credentials: map[string]any{"model_mapping": map[string]any{"gemini-2.5-pro": "gemini-2.5-pro"}},
+			},
+			{
+				ID:          2,
+				Platform:    PlatformGemini,
+				Type:        AccountTypeAPIKey,
+				Priority:    2,
+				Status:      StatusActive,
+				Schedulable: true,
+				Credentials: map[string]any{"model_mapping": map[string]any{"gemini-2.5-flash": "gemini-2.5-flash"}},
+			},
+		},
+		accountsByID: map[int64]*Account{},
+	}
+	for i := range repo.accounts {
+		repo.accountsByID[repo.accounts[i].ID] = &repo.accounts[i]
+	}
+
+	cache := &mockGatewayCacheForPlatform{}
+
+	svc := &GatewayService{
+		accountRepo: repo,
+		cache:       cache,
+		cfg:         testConfig(),
+	}
+
+	acc, err := svc.selectAccountForModelWithPlatform(ctx, nil, "", "gemini-2.5-flash", nil, PlatformGemini)
+	require.NoError(t, err)
+	require.NotNil(t, acc)
+	require.Equal(t, int64(2), acc.ID, "应过滤不支持请求模型的 APIKey 账号")
+
+	acc, err = svc.selectAccountForModelWithPlatform(ctx, nil, "", "gemini-3-pro-preview", nil, PlatformGemini)
+	require.Error(t, err)
+	require.Nil(t, acc)
+	require.Contains(t, err.Error(), "supporting model")
+}
+
 func TestGatewayService_SelectAccountForModelWithPlatform_StickyInGroup(t *testing.T) {
 	ctx := context.Background()
 	groupID := int64(50)
@@ -1070,6 +1119,36 @@ func TestGatewayService_isModelSupportedByAccount(t *testing.T) {
 			model:    "claude-3-5-sonnet-20241022",
 			expected: true,
 		},
+		{
+			name:     "Gemini平台-无映射配置-支持所有模型",
+			account:  &Account{Platform: PlatformGemini, Type: AccountTypeAPIKey},
+			model:    "gemini-2.5-flash",
+			expected: true,
+		},
+		{
+			name: "Gemini平台-有映射配置-只支持配置的模型",
+			account: &Account{
+				Platform: PlatformGemini,
+				Type:     AccountTypeAPIKey,
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{"gemini-2.5-pro": "gemini-2.5-pro"},
+				},
+			},
+			model:    "gemini-2.5-flash",
+			expected: false,
+		},
+		{
+			name: "Gemini平台-有映射配置-支持配置的模型",
+			account: &Account{
+				Platform: PlatformGemini,
+				Type:     AccountTypeAPIKey,
+				Credentials: map[string]any{
+					"model_mapping": map[string]any{"gemini-2.5-pro": "gemini-2.5-pro"},
+				},
+			},
+			model:    "gemini-2.5-pro",
+			expected: true,
+		},
 	}
 
 	for _, tt := range tests {

backend/internal/service/gateway_service.go

@@ -2825,10 +2825,6 @@ func (s *GatewayService) isModelSupportedByAccount(account *Account, requestedMo
 	if account.Platform == PlatformAnthropic && account.Type != AccountTypeAPIKey {
 		requestedModel = claude.NormalizeModelID(requestedModel)
 	}
-	// Gemini API Key 账户直接透传，由上游判断模型是否支持
-	if account.Platform == PlatformGemini && account.Type == AccountTypeAPIKey {
-		return true
-	}
 	// 其他平台使用账户的模型支持检查
 	return account.IsModelSupported(requestedModel)
 }