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/src/tools/handlers/multi_agents_tests.rs

@@ -2239,6 +2239,7 @@ async fn send_input_accepts_structured_items() {
         .expect("send_input should succeed");
 
     let expected = Op::UserInput {
+        environments: None,
         items: vec![
             UserInput::Mention {
                 name: "drive".to_string(),