Problem: previously, the deployment-wide chat template allowlist was never actually wired in from `chatd.go`
- Extracts `parseChatTemplateAllowlist` into shared `coderd/util/xjson.ParseUUIDList`
- Adds `Server.chatTemplateAllowlist()` method that reads the allowlist from DB
- Passes `AllowedTemplateIDs` callback to `ListTemplates`, `ReadTemplate`, and `CreateWorkspace` tool constructors
> 🤖 Created by Coder Agents and reviewed by a human.
Cian Johnston