Add proactive message update example

[singhk97]

Summary

• Add Example 3 to the proactive-messaging sample showing how to send a message and then update it using the ConversationClient
• Demonstrates the send-then-update pattern for proactive messaging without a running server

Test plan

• Run uv run src/main.py <conversation_id> and verify all 3 examples work
• Confirm the third message shows "Status: Pending..." then updates to "Status: Complete"

[Copilot]

Pull request overview

Adds a third scenario to the proactive-messaging example app to demonstrate the “send then update” proactive messaging pattern using the SDK’s Conversations activity update API (no HTTP server required).

Changes:

• Convert the previous module-level narrative docstring into a comment header (license-only module docstring remains).
• Add an Example 3 function that sends a proactive message, waits briefly, then updates the same activity via app.api.conversations.activities(...).update(...).
• Wire Example 3 into main() with a short delay between examples.

[lilyydu]

Nice! Thanks for updating!!