Add force_base_channel support to conversation references for Copilot Web OAuth flows

[Copilot]

M365 Copilot Web can send composite channelId values (for example, msteams:copilot-web), but OAuth token exchange must use the base channel. This change ports the .NET behavior by adding an opt-in base-channel mode and wiring OAuth flow state construction to use it.

• Activity conversation reference API

  • Extended Activity.get_conversation_reference() with optional force_base_channel: bool | None = None.
  • When enabled, the method emits the base channel by splitting on the first : (preserving existing behavior when unset/false).

• OAuth flow token-exchange path

  • Updated _OAuthFlow.begin_flow() to build TokenExchangeState.conversation with:
    • activity.get_conversation_reference(force_base_channel=True)
  • This ensures token exchange uses channel-compatible conversation metadata for Copilot Web traffic.

• Focused coverage for new behavior

  • Added/updated tests to cover:
    • base-channel extraction for composite and multi-segment channel IDs
    • non-forced behavior retaining full channel ID
    • OAuth flow invocation using force_base_channel=True

# activity.py
conversation = activity.get_conversation_reference(force_base_channel=True)
# "msteams:copilot-web" -> "msteams"

[Copilot]

Pull request overview

Adds opt-in base-channel conversation references so OAuth state can handle composite Copilot Web channel IDs.

Changes:

• Added force_base_channel to Activity.get_conversation_reference().
• Updated OAuth flow state construction to request a base-channel conversation reference.
• Added focused tests for base-channel extraction and OAuth usage.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
libraries/microsoft-agents-activity/microsoft_agents/activity/activity.py	Adds base-channel extraction support to conversation references.
libraries/microsoft-agents-hosting-core/microsoft_agents/hosting/core/_oauth/_oauth_flow.py	Uses the new option when building OAuth token exchange state.
tests/activity/test_activity.py	Adds tests for forced and non-forced channel reference behavior.
tests/hosting_core/_oauth/test_oauth_flow.py	Updates OAuth test setup and verifies the forced-base call is made.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

tests/hosting_core/_oauth/test_oauth_flow.py:250

• TokenExchangeState does not expose a top-level channel_id; the channel lives under its conversation field. These assertions will raise AttributeError when the spy captures the state instance instead of validating the encoded state's conversation channel.

        encoded_state_instance = get_encoded_state_spy.call_args.args[0]
        assert encoded_state_instance.channel_id == base_channel_id
        assert encoded_state_instance.channel_id != composite_channel_id