Feature request: show sessions from all providers in the sidebar and make model switching provider-aware

Summary

When multiple model providers are configured, Codex Desktop / sidebar history appears to be filtered by the currently active provider. That makes sessions created under another provider disappear from the sidebar, even though the underlying local thread data still exists.

This is not just a history-visibility issue. The model switching flow should also be provider-aware so a user can browse and resume sessions from different providers without them being hidden by the active provider.

Requested behavior

1. Sidebar / recent sessions should show local sessions from all providers by default.
2. Provider filtering should be explicit UI state, not an implicit consequence of the active model provider.
3. Model switching should let users resume a thread with its recorded provider, or clearly switch between compatible providers.
4. If a thread cannot be resumed under the current provider, the UI should explain that explicitly instead of hiding the thread.

Repro

1. Configure two providers, for example the default OpenAI subscription backend and a custom provider.
2. Create sessions under provider A.
3. Switch the active provider to B.
4. Open the sidebar / history view.
5. Sessions from provider A are missing or partially hidden.

Why this matters

Users with mixed provider setups need the sidebar to be a unified local session view. Otherwise switching providers makes older sessions look lost, even though they still exist locally.

Related issues

• #20184
• #31625
• #24197

Note

I verified locally that thread/list can return all providers when the request uses the all-provider form of the filter, so this looks like a UI/query-default problem rather than a data-loss issue.