fix: pretty-print the sandbox config in the TUI/exec modes (#1376)

Now that https://github.com/openai/codex/pull/1373 simplified the
sandbox config, we can print something much simpler in the TUI (and in
`codex exec`) to summarize the sandbox config.

Before:

![Screenshot 2025-06-24 at 5 45
52 PM](https://github.com/user-attachments/assets/b7633efb-a619-43e1-9abe-7bb0be2d0ec0)

With this change:

![Screenshot 2025-06-24 at 5 46
44 PM](https://github.com/user-attachments/assets/8d099bdd-a429-4796-a08d-70931d984e4f)

For reference, my `config.toml` contains:

```
[sandbox]
mode = "workspace-write"
writable_roots = ["/tmp", "/Users/mbolin/.pyenv/shims"]
```

Fixes https://github.com/openai/codex/issues/1248

codex-rs/exec/src/event_processor.rs

@@ -1,4 +1,5 @@
 use codex_common::elapsed::format_elapsed;
+use codex_common::summarize_sandbox_policy;
 use codex_core::WireApi;
 use codex_core::config::Config;
 use codex_core::model_supports_reasoning_summaries;
@@ -134,7 +135,7 @@ impl EventProcessor {
             ("model", config.model.clone()),
             ("provider", config.model_provider_id.clone()),
             ("approval", format!("{:?}", config.approval_policy)),
-            ("sandbox", format!("{:?}", config.sandbox_policy)),
+            ("sandbox", summarize_sandbox_policy(&config.sandbox_policy)),
         ];
         if config.model_provider.wire_api == WireApi::Responses
             && model_supports_reasoning_summaries(&config.model)