Fix App Send Targeted Messages by updating ConversationReference User

[ShanmathiMayuramKrithivasan]

Problem

App.Send() cannot send targeted messages because it builds ConversationReference, dropping the required User field that specifies the recipient id for targeted messages.

Fix

• Set ConversationReference.User = activity.Recipient when isTargeted=true in App.Send()
• This ensures the sender plugin can properly set the outgoing activity's recipient
• Maintains backward compatibility for non-targeted messages (User remains null)

[Copilot]

Pull request overview

This PR fixes the App's Send method to properly support targeted messages by ensuring the ConversationReference User field is set when sending targeted activities. The fix addresses the issue where targeted messages weren't being delivered to the correct recipient because the User field in the ConversationReference was not being populated.

Key changes:

• Added validation to ensure activity.Recipient is provided when sending targeted messages
• Set ConversationReference.User to activity.Recipient for targeted messages, which enables proper message routing through the sender plugin

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[Copilot]

The new validation logic for targeted messages lacks test coverage. Consider adding tests to verify:

1. An ArgumentException is thrown when isTargeted is true but activity.Recipient is null
2. The validation passes when both isTargeted is true and activity.Recipient is set
3. No exception is thrown when isTargeted is false regardless of activity.Recipient value

These tests would ensure the validation behaves correctly under all scenarios.

[Copilot]

The new User field assignment logic lacks test coverage. Consider adding tests to verify:

1. When isTargeted is true, reference.User is set to activity.Recipient
2. When isTargeted is false, reference.User is set to null
3. The ConversationReference is passed correctly to the sender plugin with the User field properly set

This would ensure that targeted messages are routed to the correct recipient.