[codex] support executor registry remote environments (#21323)

## 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`

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -6,6 +6,7 @@ mod attestation;
 mod collaboration_mode;
 mod command_exec;
 mod config;
+mod environment;
 mod experimental_feature;
 mod feedback;
 mod fs;
@@ -31,6 +32,7 @@ pub use attestation::*;
 pub use collaboration_mode::*;
 pub use command_exec::*;
 pub use config::*;
+pub use environment::*;
 pub use experimental_feature::*;
 pub use feedback::*;
 pub use fs::*;