Added user-managed API keys for enterprise-configured providers (#919)

Co-authored-by: Thorsten Sommer <SommerEngineering@users.noreply.github.com>

app/MindWork AI Studio/wwwroot/changelog/v26.8.2.md

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 # v26.8.2, build 252 (2026-08-xx xx:xx UTC)
+- Added providers for which you bring your own API key. Until now, a provider your organization configured had to come with a shared API key, which meant your IT department needed one key for everybody. Such a provider can now be handed out without a key, so that everyone signs in with their own, for example, a personal OpenAI or Anthropic account. This works for chat, embedding, and transcription providers alike. The provider stays managed by your organization: the host, the model, the instance name, and everything else remain fixed, and the only thing you can edit is the API key. In the settings, these providers carry a key icon instead of the usual lock, so you can see at a glance where you have to add your key; for embedding providers, the test button stays available, so you can check your key right after entering it. Your key is stored on your device in the operating system's credential store, and it stays there even when your organization withdraws the provider later, so it is still in place should the same provider return. For IT departments: the new `AllowUserProvidedAPIKey` option does this, and it works for `LLM_PROVIDERS`, `EMBEDDING_PROVIDERS`, and `TRANSCRIPTION_PROVIDERS` alike.
+- Fixed a removed API key staying in the operating system's credential store. When you cleared the API key of a provider, the previous key remained stored and was still used. It is now removed together with your change.