Migrate exec-server remote registration to environments (#23633)

## Summary
- migrate exec-server remote registration naming from executor to
environment
- align CLI, public Rust exports, registry error messages, and relay
test fixtures with the environment registry contract
- keep the live registration path and response model consistent with
`/cloud/environment/{environment_id}/register`

## Verification
- `cargo test -p codex-exec-server
remote::tests::register_environment_posts_with_auth_provider_headers
--manifest-path /Users/richardlee/code/codex/codex-rs/Cargo.toml`
- `cargo test -p codex-exec-server --test relay
multiplexed_remote_environment_routes_independent_virtual_streams
--manifest-path /Users/richardlee/code/codex/codex-rs/Cargo.toml`
- `cargo check -p codex-cli --manifest-path
/Users/richardlee/code/codex/codex-rs/Cargo.toml` (still running when PR
opened; will update after completion if needed)

codex-rs/file-system/src/lib.rs

@@ -130,7 +130,7 @@ fn file_system_policy_has_cwd_dependent_entries(
 pub type FileSystemResult<T> = io::Result<T>;
 
 /// Abstract filesystem access used by components that may operate locally or via
-/// a remote executor.
+/// a remote environment.
 #[async_trait]
 pub trait ExecutorFileSystem: Send + Sync {
     async fn read_file(