Add force_base_channel option to get_conversation_reference for M365 Copilot Web OAuth

[MattB-msft]

Summary

Port of Agents-for-net#828 to Python.

The .NET SDK added a forceBaseChannel parameter to GetConversationReference() to fix OAuth sign-in behavior in M365 Copilot Web (see Agents-for-net#827). When M365 Copilot Web sends activities, the channelId is a composite identifier. For OAuth token exchange, the base channel must be used instead.

Changes Needed

libraries/microsoft-agents-activity/microsoft_agents/activity/activity.py

• Add optional force_base_channel: bool | None = None parameter to get_conversation_reference()
• When True, use the base channel (e.g. self.channel_id.channel or equivalent) instead of the raw channel_id string

libraries/microsoft-agents-hosting-core/microsoft_agents/hosting/core/connector/client/user_token_client.py

• Update OAuth token exchange code paths to call get_conversation_reference(force_base_channel=True) — matching the behavior added to RestUserTokenClient.cs in the .NET PR

Reference

• .NET fix: Add forceBaseChannel option to GetConversationReference Agents-for-net#828
• Root issue: OAuth sign-in shows magic-code page in M365 Copilot Web — IUserTokenClient leaks {channelId}:{productInfo} to BF Token Service Agents-for-net#827