In local projects, initialize the list of agents in the agent server
store immediately. Previously we were initializing the list only after a
delay, in an attempt to avoid sending the `ExternalAgentsUpdated`
message to the downstream client (if any) before its handlers were
initialized. But we already have a separate codepath for that situation,
in the `AgentServerStore::shared`, and we can insert the delay in that
place instead.
Release Notes:
- acp: Fixed a bug where starting an external agent thread soon after
Zed starts up would show a "not registered" error.
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Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
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