Add CompatTeamsSSOTokenExchangeMiddleware

[rido-min]

Summary

• Adds CompatTeamsSSOTokenExchangeMiddleware as a drop-in replacement for the BF SDK's TeamsSSOTokenExchangeMiddleware that works with the Teams Bot Core SDK via CompatUserTokenClient
• Fixes System.InvalidCastException: Unable to cast object of type 'CompatConversations' to type 'Microsoft.Bot.Connector.Conversations' thrown when BF SDK's TeamsInfo.GetMemberAsync is invoked through TeamsActivityHandler.OnTeamsMembersAddedDispatchAsync
• Calling code should use CompatTeamsSSOTokenExchangeMiddleware + CompatTeamsInfo instead of the BF SDK equivalents

Error details

System.InvalidCastException
  Message=Unable to cast object of type 'Microsoft.Teams.Bot.Compat.CompatConversations'
          to type 'Microsoft.Bot.Connector.Conversations'.
  StackTrace:
   at Microsoft.Bot.Builder.Teams.TeamsInfo.<GetMemberAsync>d__22.MoveNext()
   at Microsoft.Bot.Builder.Teams.TeamsActivityHandler.<OnTeamsMembersAddedDispatchAsync>d__27.MoveNext()
   at Microsoft.Bot.Builder.ActivityHandler.<OnTurnAsync>d__0.MoveNext()
   at Microsoft.Bot.Builder.Teams.TeamsSSOTokenExchangeMiddleware.<OnTurnAsync>d__3.MoveNext()

The BF SDK's TeamsInfo.GetMemberAsync casts IConversations to the concrete Conversations class. Since CompatConversations only implements IConversations (not extending the concrete class), this cast fails. The compat middleware avoids triggering BF SDK code paths that perform this cast.

Test plan

• Register CompatTeamsSSOTokenExchangeMiddleware in place of TeamsSSOTokenExchangeMiddleware in an SSO sample bot
• Use CompatTeamsInfo instead of TeamsInfo in bot handlers
• Verify SSO token exchange flow completes without InvalidCastException
• Verify deduplication logic works for multi-client signin scenarios

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