removing network proxy for yolo (#17742)

**Summary**
- prevent managed requirements.toml network settings from leaking into
DangerFullAccess / yolo turns by gating managed proxy attachment on
sandbox mode
- keep guardian/sandboxed modes on the managed proxy path, while making
true yolo bypass the proxy entirely, including /shell full-access
commands

codex-rs/core/src/tasks/user_shell.rs

@@ -163,7 +163,9 @@ pub(crate) async fn execute_user_shell_command(
         cwd: cwd.clone(),
         env: exec_env_map,
         exec_server_env_config: None,
-        network: turn_context.network.clone(),
+        // `/shell` is the explicit full-access escape hatch, so it must not
+        // inherit a managed proxy from the surrounding session or turn.
+        network: None,
         // TODO(zhao-oai): Now that we have ExecExpiration::Cancellation, we
         // should use that instead of an "arbitrarily large" timeout here.
         expiration: USER_SHELL_TIMEOUT_MS.into(),