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/tool_harness.rs

@@ -78,6 +78,7 @@ async fn shell_tool_executes_command_and_streams_output() -> anyhow::Result<()>
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please run the shell command".into(),
                 text_elements: Vec::new(),
@@ -149,6 +150,7 @@ async fn update_plan_tool_emits_plan_update_event() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please update the plan".into(),
                 text_elements: Vec::new(),
@@ -230,6 +232,7 @@ async fn update_plan_tool_rejects_malformed_payload() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please update the plan".into(),
                 text_elements: Vec::new(),
@@ -326,6 +329,7 @@ async fn apply_patch_tool_executes_and_emits_patch_events() -> anyhow::Result<()
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please apply a patch".into(),
                 text_elements: Vec::new(),
@@ -430,6 +434,7 @@ async fn apply_patch_reports_parse_diagnostics() -> anyhow::Result<()> {
 
     codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "please apply a patch".into(),
                 text_elements: Vec::new(),