Split API listings by target framework and namespace

[Copilot]

The generated API listings had grown into large monolithic files per target framework. This changes the export flow to keep framework separation while splitting each listing into namespace-scoped files.

• API export flow

  • GenAPI now writes temporary monolithic outputs to artifacts/api/.
  • scripts/Export-Api.ps1 owns the final api/ layout.

• Namespace split

  • The export script cleans and normalizes GenAPI output as before.
  • It dynamically discovers OpenAI namespaces and writes one file per namespace under the target framework folder.

• Generated layout

  api/net10.0/OpenAI.net10.0.cs
  api/net10.0/OpenAI.Assistants.net10.0.cs
  api/net10.0/OpenAI.Audio.net10.0.cs
  api/net10.0/OpenAI.Chat.net10.0.cs
  

• Docs

  • Updated api/README.md to describe the target-framework and namespace-based layout.