Wire up UserAgent from config to HTTP requests

[Copilot]

KubernetesClientConfiguration.UserAgent existed as a property but was never applied to outgoing HTTP requests.

• Default value: kubernetes-csharp/{GeneratedApiVersion.AssemblyVersion} so all requests identify the client out of the box
• Applied in CreateHttpClient: Sets HttpClient.DefaultRequestHeaders via TryAddWithoutValidation to tolerate version strings with metadata (e.g. + suffix)
• Overridable: Users can set UserAgent on the config to any custom string, or null/empty to omit

var config = new KubernetesClientConfiguration
{
    Host = "https://my-cluster",
    UserAgent = "my-controller/2.0",
};
var client = new Kubernetes(config);

Tests added for both default and custom user-agent propagation.

[Copilot]

Pull request overview

Adds a default HTTP User-Agent for the Kubernetes client and verifies the header behavior via integration-style tests against the mock API server.

Changes:

• Set a default KubernetesClientConfiguration.UserAgent value to kubernetes-csharp/{GeneratedApiVersion.AssemblyVersion}.
• Apply the configured UserAgent to the underlying HttpClient default request headers.
• Add tests covering both default and custom User-Agent behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/KubernetesClient.Tests/AuthTests.cs	Adds tests asserting default and custom User-Agent headers are sent.
src/KubernetesClient/KubernetesClientConfiguration.cs	Introduces a default UserAgent value on the configuration object.
src/KubernetesClient/Kubernetes.ConfigInit.cs	Applies the configured UserAgent to HttpClient.DefaultRequestHeaders.

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[tg123]

@copilot fix build

[Copilot]

@copilot fix build

The MSBuild build (codgen job) was failing because .NET SDK 10.0.201 (latest 10.0.x) requires MSBuild 18.0, but VS2022 only ships MSBuild 17.x. This is a pre-existing CI issue unrelated to the user-agent changes — all dotnet build jobs and e2e were passing.

Fixed in 1195e86 by pinning the codegen job to 10.0.1xx SDK feature band, which is compatible with VS2022's MSBuild 17.x.

[tg123]

/LGTM

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Copilot, tg123

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