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/search_tool.rs

@@ -502,6 +502,7 @@ async fn tool_search_returns_deferred_tools_without_follow_up_tool_injection() -
     let test = builder.build(&server).await?;
     test.codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Find the calendar create tool".to_string(),
                 text_elements: Vec::new(),
@@ -778,6 +779,7 @@ async fn tool_search_returns_deferred_dynamic_tool_and_routes_follow_up_call() -
 
     test.codex
         .submit(Op::UserInput {
+            environments: None,
             items: vec![UserInput::Text {
                 text: "Use the automation tool".to_string(),
                 text_elements: Vec::new(),