feat: support allowed_sandbox_modes in requirements.toml (#8298)

This adds support for `allowed_sandbox_modes` in `requirements.toml` and
provides legacy support for constraining sandbox modes in
`managed_config.toml`. This is converted to `Constrained<SandboxPolicy>`
in `ConfigRequirements` and applied to `Config` such that constraints
are enforced throughout the harness.

Note that, because `managed_config.toml` is deprecated, we do not add
support for the new `external-sandbox` variant recently introduced in
https://github.com/openai/codex/pull/8290. As noted, that variant is not
supported in `config.toml` today, but can be configured programmatically
via app server.

codex-rs/common/src/config_summary.rs

@@ -10,7 +10,10 @@ pub fn create_config_summary_entries(config: &Config, model: &str) -> Vec<(&'sta
         ("model", model.to_string()),
         ("provider", config.model_provider_id.clone()),
         ("approval", config.approval_policy.value().to_string()),
-        ("sandbox", summarize_sandbox_policy(&config.sandbox_policy)),
+        (
+            "sandbox",
+            summarize_sandbox_policy(config.sandbox_policy.get()),
+        ),
     ];
     if config.model_provider.wire_api == WireApi::Responses {
         let reasoning_effort = config