Resumig a session and exiting immediately without interacting deletes the session entirely

### What happened?

When I resume a session and (for whatever reason) exit the session without issuing any commands, I can no longer resume that same session - the entire session history is lost. I cannot invoke `gemini --resume` with the same UUID, the entry in the --list-sessions output disappears.

### What did you expect to happen?

I expect to still be able to `gemini --resume ` of my session after previously resuming it but exiting without issuing any instructions.

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
> /about
│ CLI Version 0.58.0 │
│ Git Commit ac9431c9e │
│ Model Auto │
│ Sandbox no sandbox │
│ OS linux │
│ Auth Method Signed in with Google (zach.coffin@vetsource.com) │
│ Tier Gemini Code Assist Enterprise │
│ GCP Project vetsource-clouddevstacks │
│ IDE Client VS Code │
```

### Login information

Google account

### Anything else we need to know?

_No response_

Reproduce the issue by resuming a session with `gemini --resume `, exiting without entering a command, and checking `gemini --list-sessions`. Start by tracing the CLI's session-resume and exit handling. Done means the resumed session remains listed and can be opened again with the same UUID after an immediate exit.

Written by the indexing model from the issue text.