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/app-server/tests/suite/v2/turn_start.rs

@@ -1733,6 +1733,7 @@ async fn turn_start_updates_sandbox_and_cwd_between_turns_v2() -> Result<()> {
     // first turn with workspace-write sandbox and first_cwd
     let first_turn = mcp
         .send_turn_start_request(TurnStartParams {
+            environments: None,
             thread_id: thread.id.clone(),
             input: vec![V2UserInput::Text {
                 text: "first turn".to_string(),
@@ -1773,6 +1774,7 @@ async fn turn_start_updates_sandbox_and_cwd_between_turns_v2() -> Result<()> {
     // second turn with workspace-write and second_cwd, ensure exec begins in second_cwd
     let second_turn = mcp
         .send_turn_start_request(TurnStartParams {
+            environments: None,
             thread_id: thread.id.clone(),
             input: vec![V2UserInput::Text {
                 text: "second turn".to_string(),