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/realtime_conversation.rs

@@ -1896,6 +1896,7 @@ async fn conversation_user_text_turn_is_sent_to_realtime_when_active() -> Result
     let prefixed_user_text = format!("[USER] {user_text}");
     test.codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: user_text.to_string(),
                 text_elements: Vec::new(),
@@ -2025,6 +2026,7 @@ async fn conversation_user_text_turn_is_capped_when_mirrored_to_realtime() -> Re
     );
     test.codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: user_text.clone(),
                 text_elements: Vec::new(),
@@ -3183,6 +3185,7 @@ async fn inbound_handoff_request_steers_active_turn() -> Result<()> {
 
     test.codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "first prompt".to_string(),
                 text_elements: Vec::new(),