BuildDefaultConfig should support specifying a context when using multi-file KUBECONFIG

Is your feature request related to a problem?

When KUBECONFIG is set to multiple files (e.g., ~/.kube/config:/home/user/.kube/other-config on Linux or C:\Users\me\.kube\config;C:\Users\me\.kube\other on Windows), the only method that correctly merges them is BuildDefaultConfig(). However, BuildDefaultConfig() doesn't accept a currentContext parameter — it always uses the current-context from the merged config.

If you need to specify a particular context (common in test harnesses, multi-cluster tools, and CI systems), the only option is BuildConfigFromConfigFile(string kubeconfigPath, string currentContext). But this method wraps the path in a single FileInfo without splitting on the platform delimiter, so it fails with a file-not-found error when given a multi-file KUBECONFIG value.

There's no way to do both, which kubectl supports natively via --context combined with multi-file KUBECONFIG.

Describe the solution you'd like

A way to build a KubernetesClientConfiguration that both merges multiple KUBECONFIG files and allows specifying a context — matching what kubectl supports natively with KUBECONFIG=a:b kubectl --context=my-context.

Environment

• KubernetesClient NuGet version: 18.x
• Platform: Windows and Linux
• Use case: Test harnesses that need to target a specific cluster context in a multi-file KUBECONFIG environment

Happy to open an PR if you indicate the preferred approach i.e. add an overload to BuildDefaultConfig

Start by locating the BuildDefaultConfig and BuildConfigFromConfigFile entry points and trace how KUBECONFIG paths and currentContext are handled. Compare the configuration behavior needed for platform-delimited multi-file values with kubectl's context selection. Done means callers can merge multiple KUBECONFIG files while selecting an explicit context on Windows and Linux.