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/exec/src/lib.rs

@@ -745,6 +745,7 @@ async fn run_exec_session(args: ExecRunArgs) -> anyhow::Result<()> {
                         thread_id: primary_thread_id_for_span.clone(),
                         input: items.into_iter().map(Into::into).collect(),
                         responsesapi_client_metadata: None,
+                        environments: None,
                         cwd: Some(default_cwd),
                         approval_policy: Some(default_approval_policy.into()),
                         approvals_reviewer: None,