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/pending_input.rs

@@ -95,6 +95,7 @@ async fn build_codex(server: &StreamingSseServer) -> Arc<CodexThread> {
 async fn submit_user_input(codex: &CodexThread, text: &str) {
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: text.to_string(),
                 text_elements: Vec::new(),
@@ -109,6 +110,7 @@ async fn submit_user_input(codex: &CodexThread, text: &str) {
 async fn submit_danger_full_access_user_turn(test: &TestCodex, text: &str) {
     test.codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: text.to_string(),
                 text_elements: Vec::new(),
@@ -272,6 +274,7 @@ async fn injected_user_input_triggers_follow_up_request_with_deltas() {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "first prompt".into(),
                 text_elements: Vec::new(),
@@ -289,6 +292,7 @@ async fn injected_user_input_triggers_follow_up_request_with_deltas() {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "second prompt".into(),
                 text_elements: Vec::new(),