test: Improve cmd/node-doctor createExporters coverage from 29.8% to 61.7%

Add 9 new test functions for createExporters function:
- TestCreateExporters_HTTPExporterEnabled
- TestCreateExporters_PrometheusExporterEnabled
- TestCreateExporters_HealthServerEnabled
- TestCreateExporters_MultipleExportersEnabled
- TestCreateExporters_HTTPExporterWithValidConfig
- TestCreateExporters_PrometheusWithMetricsPath
- TestCreateExporters_HTTPWithRetryConfig
- TestCreateExporters_HealthServerWithCustomPaths
- TestCreateExporters_ExporterCleanup

Coverage target of 80%+ not achievable due to:
- HTTP exporter success path requires valid kubeconfig/auth config
- Kubernetes exporter requires actual kubeconfig
- Would require integration tests or extensive mocking

Task: #3856

Author: mattmattox