feat: add support for ctx.delete

[lilyydu]

partially resolves: #338
old pr: #343 (abandoned, had to cherry pick due to removal of prev commits)

previously, the only way to delete an activity was directly calling
await app.api.conversations.activities(conversation_reference.conversation.id).delete(conversation_reference.activity_id)

this wasn't very consistent with our flow for sending and updating activities (via send). hence, we are now adding ctx.delete(activity_id) to ActivityContext for deleting a bot activity.

NOTE: We previously had MessageDeleteActivityInput but this was an invalid type on POST/PUT. Delete is a separate verb and needs its own path.

This only partially resolves the issue named above. In addition to this change, I think we should make it more explicit in our docs.

Details:

• Proactive deletion supported via optional conversation_ref (same pattern as ctx.send())
• Targeted activities supported via targeted=True (experimental, emits ExperimentalTeamsTargeted warning)
• Routes to DELETE /v3/conversations/{id}/activities/{activityId} — no body, success = no exception