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/tests/suite/resume_warning.rs

@@ -24,7 +24,7 @@ fn resume_history(
     let turn_ctx = TurnContextItem {
         cwd: config.cwd.clone(),
         approval_policy: config.approval_policy.value(),
-        sandbox_policy: config.sandbox_policy.clone(),
+        sandbox_policy: config.sandbox_policy.get().clone(),
         model: previous_model.to_string(),
         effort: config.model_reasoning_effort,
         summary: config.model_reasoning_summary,