models_dev.py's fetch uses a synchronous requests.get(timeout=15). Called
from the async gateway message handlers, it blocked the event loop for up
to 15s, starving Discord heartbeats and causing ClientConnectionResetError
disconnects.
Adds get_model_context_length_async() which offloads the entire sync
resolution chain to a worker thread via asyncio.to_thread(), and switches
the two async gateway call sites (_prepare_inbound_message_text,
_handle_message_with_agent) to await it. The loop stays responsive; the
sync path remains the single source of truth for the cache.
Salvaged from PR #22753 by @itenev. Follow-up: dropped the unused
fetch_models_dev_async/lookup_models_dev_context_async aiohttp variants
from the original PR (dead code with zero callers that had drifted from
the sync cache logic) — the to_thread wrapper already runs the sync path
off-loop, so they were redundant.
f981d47cb0