[codex] support executor registry remote environments

[miz-openai]

Summary

Support registry-backed remote executors end to end so downstream services can resolve an executor id into an exec-server URL and make that environment available to Codex without relying on the legacy cloud environments flow.

What changed

• switch remote executor registration to the executor registry bootstrap contract
• allow named remote environments to be inserted into EnvironmentManager at runtime
• add the experimental app-server RPC environment/add so initialized experimental clients can register those remote environments for later thread/start and turn/start selection

Validation

Ran focused validation locally:

• cargo test -p codex-exec-server environment_manager_
• cargo test -p codex-exec-server register_executor_posts_with_bearer_token_header
• cargo test -p codex-app-server-protocol