Add cached environment manager for exec server URL (#15785)

Add environment manager that is a singleton and is created early in
app-server (before skill manager, before config loading).

Use an environment variable to point to a running exec server.

scripts/run_tui_with_exec_server.sh

@@ -0,0 +1,61 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cargo_root="$repo_root/codex-rs"
+listen_url="${CODEX_EXEC_SERVER_LISTEN_URL:-ws://127.0.0.1:0}"
+start_timeout_seconds="${CODEX_EXEC_SERVER_START_TIMEOUT_SECONDS:-120}"
+tmp_dir="$(mktemp -d "${TMPDIR:-/tmp}/codex-tui-with-exec-server.XXXXXX")"
+stdout_log="$tmp_dir/exec-server.stdout"
+stderr_log="$tmp_dir/exec-server.stderr"
+server_pid=""
+exec_server_url=""
+
+cleanup() {
+  if [[ -n "$server_pid" ]]; then
+    kill "$server_pid" >/dev/null 2>&1 || true
+    wait "$server_pid" >/dev/null 2>&1 || true
+  fi
+  rm -rf "$tmp_dir"
+}
+
+trap cleanup EXIT INT TERM HUP
+
+(
+  cd "$cargo_root"
+  cargo run -p codex-exec-server -- --listen "$listen_url"
+) >"$stdout_log" 2>"$stderr_log" &
+server_pid="$!"
+
+# Wait for the server to print its bound websocket URL before launching the TUI.
+for _ in $(seq 1 "$((start_timeout_seconds * 20))"); do
+  if [[ -s "$stdout_log" ]]; then
+    exec_server_url="$(head -n 1 "$stdout_log" | tr -d '\r')"
+    if [[ "$exec_server_url" == ws://* ]]; then
+      break
+    fi
+  fi
+
+  if ! kill -0 "$server_pid" >/dev/null 2>&1; then
+    cat "$stderr_log" >&2 || true
+    cat "$stdout_log" >&2 || true
+    echo "failed to start codex-exec-server" >&2
+    exit 1
+  fi
+
+  sleep 0.05
+done
+
+if [[ -z "$exec_server_url" ]]; then
+  cat "$stderr_log" >&2 || true
+  cat "$stdout_log" >&2 || true
+  echo "timed out waiting ${start_timeout_seconds}s for codex-exec-server to report its websocket URL" >&2
+  exit 1
+fi
+
+export CODEX_EXEC_SERVER_URL="$exec_server_url"
+echo "Starting codex-tui with CODEX_EXEC_SERVER_URL=$CODEX_EXEC_SERVER_URL" >&2
+
+cd "$cargo_root"
+cargo run -p codex-tui --bin codex-tui -- -c features.tui_app_server=true -c mcp_oauth_credentials_store=file "$@"