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.Send() method to properly support targeted messages by ensuring the ConversationReference.User field is populated with the recipient information when sending targeted messages. Previously, this field was always null, preventing sender plugins from correctly routing messages to specific recipients.

Key Changes:

• Added validation to ensure activity.Recipient is set when isTargeted=true
• Set ConversationReference.User to activity.Recipient for targeted messages, enabling sender plugins to properly route the message
• Maintained backward compatibility by keeping User as null for non-targeted messages

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