Fix health status namespace header

[eagle27272]

Summary

• stop sending X-Vault-Namespace for GetHealthStatusAsync requests
• add an internal request helper flag that defaults to preserving existing namespace behavior for all other APIs
• add a changelog entry for the fix

Why

HashiCorp documents /sys/health as a restricted endpoint and states that clients must call it from the root namespace: https://developer.hashicorp.com/vault/api-docs/system/health

When VaultClientSettings.Namespace is configured, VaultSharp currently sends X-Vault-Namespace on health checks, which makes the request target a namespace even though the endpoint is only valid at root.

Validation

• dotnet build src/VaultSharp/VaultSharp.csproj -f netstandard2.0 --no-restore

Notes

• CONTRIBUTING.MD asks contributors to run the VaultSharp.Samples console app. I did not run it here because it requires a local Vault server at http://localhost:8200 with the sample setup.