fix: wait for browser-opener process to exit before returning-W-24154138

[soridalac]

What does this PR do?

Summary

• Fixes race condition where sf org open exits before the browser-opener child process finishes launching the browser, causing intermittent
  failures — particularly on Windows and in VS Code's integrated terminal
• Awaits the child process exit instead of fire-and-forget, with a guard for the case where the process has already exited before the handler is
  registered

Test plan

• sf org open opens browser reliably on Windows (standalone terminal and VS Code integrated terminal)
• sf org open continues to work on macOS and Linux with no added delay
• sf org open --url-only is unaffected (skips browser launch)
• sf org open --browser firefox works
• sf org open --private works
• Unit tests pass

What issues does this PR fix or reference?

@W-24154138@
cli related issue: forcedotcom/cli#3646