Handle existing sessions after provider removal

**What problem would this solve?**
Several providers are already deprecated, including the Codex CLI, Gemini CLI, and Claude Code providers. These providers should eventually be removed from Goose.

Existing sessions may still reference a deprecated provider. Today, session restoration behavior can differ between the CLI, ACP server, desktop client, and Rust API. In some cases, Goose may fail with a generic error or silently fall back to another provider.

**What would a good outcome look like?**
Provider removal is handled consistently across Goose interfaces. Existing session history remains accessible, and any provider change requires an explicit user decision or use some fallback mechanism

**Possible approaches**
Share any approaches, constraints, or trade-offs the design discussion should consider.

**Additional context**
Add any other context or screenshots about the feature request here.

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).