Problem: after restarting vim or attaching to a long-running
server, the thinking spinner did not come up while the active
session was still processing, and toggle off then on lost it
permanently. The SSE stream is event-driven on the server side:
it registers a listener at connect time and only forwards events
that fire afterwards, so a busy transition that happened before
the attach was never replayed. vim never asked for the current
state either, and toggle off wiped the only client-side copy
status_data held. Two races compounded this: sync resolving just
before set_active would wipe the freshly installed busy status
through its on_active_session_change listener, and sync resolving
before set_active would no-op entirely and leave the spinner off
indefinitely.

Solution:
- Add OpencodeApiClient:list_session_status (GET /session/status)
  and have event_manager call it on initial subscribe so a
  (re)attach actively queries current state.
- Add loading_animation.sync_from_server() called from setup().
  It caches the response on M._animation.last_status_map and only
  replays the entry for the current active session, so cross-
  session events cannot pollute the spinner.
- on_active_session_change replays from last_status_map when
  active_session is set after sync resolved, recovering without
  another round trip. It also stops clearing status_data on the
  first (nil -> X) assignment, so the freshly installed busy status
  survives the listener that fires a few ticks later.
- setup() no longer starts the spinner from preserved status_data
  (which can be stale); only the server-confirmed status starts
  it. teardown keeps status_data and status_session_id intact so
  the next setup can resume from the cached server state.
- on_session_status rejects events with no active session or a
  mismatched sessionID so SSE events from other sessions cannot
  pollute state.

This builds on the previous commit (loading_animation no longer
drives the spinner from local HTTP activity); _should_animate
remains the single source of truth, now fed by server status.