KubernetesJson use of SourceGenerationContext

[IvanJosipovic]

Summary

The client currently uses reflection-based serialization in non-AOT library even though a SourceGenerationContext is already generated.
This adds unnecessary reflection-based overhead.
Additionally, SourceGenerationContext is private and prevents us chaining it with our own custom IJsonTypeInfoResolver.

Current Behavior

1. Non-AOT library use reflection-based serialization.
2. The generated SourceGenerationContext is not used by default, even though it's in the library.
3. Callers cannot compose their own resolvers since the generated context is not exposed.

Expected Behavior

1. Non-AOT builds should use the generated SourceGenerationContext by default and fall back to reflection-based if type is missing.
2. SourceGenerationContext should be public so that we can KubernetesJson.AddJsonOptions and append our own IJsonTypeInfoResolver for our CRDs.

Rationale

• Source generation improves performance and reduces allocations even outside AOT scenarios.
• Keeps behaviour predictable and consistent between AOT and non-AOT.
• Also solves CreateNamespacedCustomObjectAsync unusable with AOT #1670

Proposed Changes

1. Initialize the client’s default JsonSerializerOptions with the generated context:

    public static class KubernetesJson
    {
        private static readonly JsonSerializerOptions JsonSerializerOptions = new JsonSerializerOptions
        {
            #if !K8S_AOT
            TypeInfoResolver = JsonTypeInfoResolver.Combine(SourceGenerationContext.Default, new DefaultJsonTypeInfoResolver()) // Uses Source Generated IJsonTypeInfoResolver when available and falls back to reflection
            #else
            TypeInfoResolver = SourceGenerationContext.Default // Uses Source Generated IJsonTypeInfoResolver
            #endif
        };
   }

2. Make SourceGenerationContext public
   This allows us to use KubernetesJson.AddJsonOptions and chain our own custom Source Generated IJsonTypeInfoResolver

     KubernetesJson.AddJsonOptions(x =>
     {
         x.TypeInfoResolver = JsonTypeInfoResolver.Combine(SourceGenerationContext.Default, MyCRDSourceGenerationContext.Default, new DefaultJsonTypeInfoResolver());
     });

3. Remove src/KubernetesClient.Aot/KubernetesJson.cs as it is now redundant.

Happy to make this PR.

Thanks!

[tg123]

Expected Behavior
looked good to me, i remember there was some issue in net7 or8, using SourceGenerationContext would cause some json issue
feel free to send pr to enable it