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/tui/src/status/tests.rs

@@ -90,12 +90,15 @@ async fn status_snapshot_includes_reasoning_details() {
     config.model_provider_id = "openai".to_string();
     config.model_reasoning_effort = Some(ReasoningEffort::High);
     config.model_reasoning_summary = ReasoningSummary::Detailed;
-    config.sandbox_policy = SandboxPolicy::WorkspaceWrite {
-        writable_roots: Vec::new(),
-        network_access: false,
-        exclude_tmpdir_env_var: false,
-        exclude_slash_tmp: false,
-    };
+    config
+        .sandbox_policy
+        .set(SandboxPolicy::WorkspaceWrite {
+            writable_roots: Vec::new(),
+            network_access: false,
+            exclude_tmpdir_env_var: false,
+            exclude_slash_tmp: false,
+        })
+        .expect("set sandbox policy");
 
     config.cwd = PathBuf::from("/workspace/tests");