Playground for testing agents, and Chat as one permanent conversation

Playground is where an organization tests its agents. Members start a conversation as their linked virtual user; owners and admins can pick another active end user. Conversations are shared across the organization, can be searched, filtered by agent or user, and archived, and each links to its session. On an agent page, Test in Playground opens setup with that agent selected.

Chat now always uses the managed Platform Chat agent. The sidebar opens your permanent conversation, New chat starts a side conversation, and All chats lists only your personal conversations. Platform Chat runs on the shell-based implementation for every organization.

See Chat and Threads and Platform Chat. PRs #4060, #4076, #4121, #4129.