If the token is still fresh (more than 2 minutes until expiry) the current
access token is returned immediately. If it is expired or within the
buffer window a refresh is triggered first.
Concurrent callers are automatically deduplicated by TokenManager.
Returns Promise<string|null>
The current valid access token, or null if refresh failed.
Ensures that the current access token is valid.
If the token is still fresh (more than 2 minutes until expiry) the current access token is returned immediately. If it is expired or within the buffer window a refresh is triggered first.
Concurrent callers are automatically deduplicated by
TokenManager.