Added a User-Agent naming the assistant for self-hosted providers

[donework]

Summary

AI Studio now sends a User-Agent header to LLM providers. This lets gateways such as LiteLLM tell AI Studio requests apart from other clients. LiteLLM shows the header as tags in its logs.

• All providers receive the app name, version, and platform:
  MindWorkAIStudio/26.9.1 (osx-arm64)
• Self-hosted providers also receive the component that sent the chat request:
  … Component/CHAT or … Component/TRANSLATION_ASSISTANT
• Assistant plugins are also named. The name is reduced to ASCII for the header, e.g. "Café Menu Writer (Beta)" becomes:
  … Component/DYNAMIC_ASSISTANT Assistant/Cafe-Menu-Writer-Beta

The assistant name only goes to self-hosted providers. Cloud providers such as OpenAI, Anthropic, or Google only receive the generic User-Agent.

Agents such as data source selection or retrieval context validation keep running as CHAT, so their requests show up as Component/CHAT.

Test checklist

• dotnet test in app/Tests passes, including the new AppUserAgentTests
• Chat via a self-hosted provider (e.g. LiteLLM) shows User-Agent: MindWorkAIStudio/… Component/CHAT in the tags
• A built-in assistant (e.g. Translation) shows Component/TRANSLATION_ASSISTANT
• An assistant plugin shows Component/DYNAMIC_ASSISTANT Assistant/<plugin name>, with umlauts and special characters cleaned up