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/core/src/config_loader/tests.rs

@@ -176,7 +176,7 @@ allowed_approval_policies = ["never", "on-request"]
     let config_requirements: ConfigRequirements = config_requirements_toml.try_into()?;
     assert_eq!(
         config_requirements.approval_policy.value(),
-        AskForApproval::OnRequest
+        AskForApproval::Never
     );
     config_requirements
         .approval_policy