Support sandboxed exec-server test environments (#40717)

## What changed

- Add a `TestCodexBuilder` entry point that accepts an explicit `TestEnv`.
- Teach the exec-server test fixture to dispatch filesystem-helper invocations.
- Allow the fixture to receive the Linux sandbox executable through
  `CODEX_TEST_LINUX_SANDBOX_EXE` and run the server on a multithreaded Tokio
  runtime.

GitOrigin-RevId: cb9c27e9db0363d4022a084bc186ccc749e25a95