Add asyncio.Lock to prevent multiple coroutines from simultaneously
refreshing the KIS access token, which hits the 1-per-minute rate
limit (EGW00133: "접근토큰 발급 잠시 후 다시 시도하세요").
Changes:
- Add self._token_lock in KISBroker.__init__
- Wrap token refresh in async with self._token_lock
- Re-check token validity after acquiring lock (double-check pattern)
- Add concurrent token refresh test (5 parallel requests → 1 API call)
The lock ensures that when multiple coroutines detect an expired token,
only the first one refreshes while others wait and reuse the result.
Fixes: #42
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
