Create a new TokenManager.
Token acquisition configuration. Falls back to env vars when omitted.
Acquire a Bot Service access token for outbound API calls.
Returns null when no credentials are configured (dev/testing mode). Caches tokens via MSAL and deduplicates concurrent requests.
null
A Bearer token string, or null if auth is not configured.
If token acquisition fails (error is also negative-cached for 30 s).
Create a new TokenManager.