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/resume.rs

@@ -86,6 +86,7 @@ async fn resume_includes_initial_messages_from_rollout_events() -> Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Record some messages".into(),
                 text_elements: text_elements.clone(),
@@ -172,6 +173,7 @@ async fn resume_includes_initial_messages_from_reasoning_events() -> Result<()>
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Record reasoning messages".into(),
                 text_elements: Vec::new(),
@@ -262,6 +264,7 @@ async fn resume_switches_models_preserves_base_instructions() -> Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Record initial instructions".into(),
                 text_elements: Vec::new(),
@@ -303,6 +306,7 @@ async fn resume_switches_models_preserves_base_instructions() -> Result<()> {
     resumed
         .codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Resume with different model".into(),
                 text_elements: Vec::new(),
@@ -319,6 +323,7 @@ async fn resume_switches_models_preserves_base_instructions() -> Result<()> {
     resumed
         .codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Second turn after resume".into(),
                 text_elements: Vec::new(),
@@ -390,6 +395,7 @@ async fn resume_model_switch_is_not_duplicated_after_pre_turn_override() -> Resu
     .await;
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Record initial instructions".into(),
                 text_elements: Vec::new(),
@@ -434,6 +440,7 @@ async fn resume_model_switch_is_not_duplicated_after_pre_turn_override() -> Resu
     resumed
         .codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "first turn after override".into(),
                 text_elements: Vec::new(),