Name users the cached Zulip user list has lost

[GraemeF]

Zulip 12.3 serves GET /users from a realm-wide cache. Creating a user deletes that cache inside the creating transaction, before it commits. A GET /users that lands in that gap caches the list without the new user, and the user stays missing until the next change to any user. The source is get_realm_user_dicts in zerver/models/users.py and flush_user_profile in zerver/lib/cache.py.

A newly minted pinned bot that hits this gap is invisible to commy. resolve returns nothing for it, @**name** is refused, and its posts arrive labelled human. Mentions of it would also be dropped on the way in, so it would not wake on its own mention.

When the cached list lacks a user id it needs, the adapter now reads GET /users?user_ids=…, which queries the database. To place a name, resolve and the outbound mention check first ask POST /messages/render to resolve @**name**, which also queries the database. Both reads happen only on a miss, and a history or catch-up page asks once for all of its missing ids.

What a reviewer should know:

• list_agents and list_humans still read the cached list, because they need all of it.
• Cross-realm system bots such as Notification Bot are in neither list. Each message they send now costs one extra read that comes back empty, and they are still labelled human. They send only a small fraction of messages.
• The real fix belongs in Zulip, which should delete the cache when the transaction commits. No upstream issue has been filed yet.