fix: apply token expiry buffer before reusing cached access token

[aaguiarz]

Summary

Credentials.getAccessTokenValue() currently reuses a cached token up to the exact expiration instant:

if (this.accessToken && (!this.accessTokenExpiryDate || this.accessTokenExpiryDate > new Date())) {
  return this.accessToken;
}

The SDK defines token-expiry buffer constants but does not use them.

Impact

A token that is close to expiry can be sent on outbound requests and expire in transit.

Expected

If token expiry is within the configured buffer window, refresh proactively instead of reusing the cached token.

Proposed Fix

Use TokenExpiryThresholdBufferInSec (+ jitter) when deciding whether cached token is still valid.