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/codex_delegate.rs

@@ -182,6 +182,7 @@ pub(crate) async fn run_codex_thread_one_shot(
 
     // Send the initial input to kick off the one-shot turn.
     io.submit(Op::UserInput {
+        environments: None,
         items: input,
         final_output_json_schema,
         responsesapi_client_metadata: None,