Maintenance

[Guldem]

1.2.0

• Added concurrent refresh protection: multiple simultaneous 401 responses now share a single refresh call instead of issuing redundant refresh requests.
• Added proactive token refresh: the interceptor now checks OAuthToken.isExpired before making the request and refreshes preemptively, avoiding unnecessary 401 round-trips.
• Added ==, hashCode, and toString() to OAuthToken for value-based equality and easier debugging.
• Exported OnErrorCallback and MemoryStorage
• Fixed exports for oauth2 package
• Updated dependencies:
  • sdk to ^3.7.0
  • meta added
  • very_good_analysis to 10.2.0