fix: prevent codex-backend from stealing originator

[owenlin0]

Why

Remote control starts by letting codex-backend initialize against the app-server as an infrastructure health/proxy client before the real remote client connects. App-server initialization also sets the process-wide originator from client_info.name, so codex-backend could become the sticky originator for later model/API requests even after the real client initialized.

What changed

• Treat codex-backend as a non-originating initialize client, alongside the existing codex_app_server_daemon probe client.
• Preserve normal per-connection initialize behavior, including session metadata and initialize analytics.
• Add regression coverage that verifies codex-backend initialize does not replace the default originator.

Testing

• cargo test -p codex-app-server --test all initialize_codex_backend_does_not_override_originator