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/prompt_caching.rs

@@ -146,6 +146,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -158,6 +159,7 @@ async fn prompt_tools_are_consistent_across_requests() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -244,6 +246,7 @@ async fn gpt_5_tools_without_apply_patch_append_apply_patch_instructions() -> an
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -256,6 +259,7 @@ async fn gpt_5_tools_without_apply_patch_append_apply_patch_instructions() -> an
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -319,6 +323,7 @@ async fn prefixes_context_and_instructions_once_and_consistently_across_requests
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -331,6 +336,7 @@ async fn prefixes_context_and_instructions_once_and_consistently_across_requests
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -413,6 +419,7 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() -> an
     // First turn
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -450,6 +457,7 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() -> an
     // Second turn after overrides
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -533,6 +541,7 @@ async fn override_before_first_turn_emits_environment_context() -> anyhow::Resul
 
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "first message".into(),
                 text_elements: Vec::new(),
@@ -685,6 +694,7 @@ async fn per_turn_overrides_keep_cached_prefix_and_key_constant() -> anyhow::Res
     // First turn
     codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -707,6 +717,7 @@ async fn per_turn_overrides_keep_cached_prefix_and_key_constant() -> anyhow::Res
     };
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -820,6 +831,7 @@ async fn send_user_turn_with_no_changes_does_not_send_environment_context() -> a
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -841,6 +853,7 @@ async fn send_user_turn_with_no_changes_does_not_send_environment_context() -> a
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),
@@ -946,6 +959,7 @@ async fn send_user_turn_with_changes_sends_environment_context() -> anyhow::Resu
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 1".into(),
                 text_elements: Vec::new(),
@@ -967,6 +981,7 @@ async fn send_user_turn_with_changes_sends_environment_context() -> anyhow::Resu
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "hello 2".into(),
                 text_elements: Vec::new(),