OpenAI API Host falls back to default when saving other fields

[bavadim]

Problem

In OpenAI provider settings, updating another field and saving can make the effective API Host fall back to the default host, even if the API Host field itself was not changed.

The UI may still show the previously entered API Host, which hides the regression.

Impact

Requests are sent to the default OpenAI endpoint instead of the user-configured host (proxy/self-hosted gateway), causing unexpected routing and failures.

Steps to Reproduce

1. Open OpenAI provider settings.
2. Set a custom value in API Host and save.
3. Reopen OpenAI settings.
4. Change any field except API Host (for example API key), then click Save.
5. Validate effective config by making a request / running provider validation and checking which host is used.

Expected

If API Host was not edited, its previously saved custom value remains the effective runtime host.

Actual

After saving changes to another field, effective host can revert to OpenAI default while UI still shows the old custom API Host text.

Scope

Provider settings save flow in desktop UI (confirmed with OpenAI API Host).