fix: 修复 CI 检查失败

- gofmt: 修复 error_passthrough_service.go 格式问题
- errcheck: 修复 error_passthrough_runtime_test.go 类型断言未检查
- staticcheck: if-else 改为 switch (gateway_service.go)
- test: 修复两个测试用例错误使用 MODEL_CAPACITY_EXHAUSTED 导致走错路径

backend/internal/service/error_passthrough_runtime_test.go

@@ -219,7 +219,9 @@ func TestApplyErrorPassthroughRule_SkipMonitoringSetsContextKey(t *testing.T) {
 	assert.True(t, matched)
 	v, exists := c.Get(OpsSkipPassthroughKey)
 	assert.True(t, exists, "OpsSkipPassthroughKey should be set when skip_monitoring=true")
-	assert.True(t, v.(bool))
+	boolVal, ok := v.(bool)
+	assert.True(t, ok, "value should be bool")
+	assert.True(t, boolVal)
 }
 
 func TestApplyErrorPassthroughRule_NoSkipMonitoringDoesNotSetContextKey(t *testing.T) {