_resolve_user_name() caches every resolved (team_id, user_id) → display
name but never evicts entries. On a long-running bot in a large
workspace every unique user the bot encounters adds a permanent entry.
Sibling structures _bot_message_ts (BOT_TS_MAX=5000) and
_assistant_threads (ASSISTANT_THREADS_MAX=5000) already have caps;
_user_name_cache had none.
Fix: add _USER_NAME_CACHE_MAX = 5000 and evict the oldest half on
overflow after each write, matching the existing sibling pattern.
Consolidates the two cache-write branches (success + API error) into a
single write so eviction runs once per resolution.
Reapplied from #23676 onto the current adapter (cache moved to
plugins/platforms/slack/adapter.py and is now keyed by
(team_id, user_id) tuples for multi-workspace safety).