Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/suite/image_rollout.rs

@@ -111,6 +111,7 @@ async fn copy_paste_local_image_persists_rollout_request_shape() -> anyhow::Resu
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![
                 UserInput::LocalImage {
                     path: abs_path.clone(),
@@ -198,6 +199,7 @@ async fn drag_drop_image_persists_rollout_request_shape() -> anyhow::Result<()>
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![
                 UserInput::Image {
                     image_url: image_url.clone(),