_LineClient's five aiohttp.ClientSession() calls omit trust_env=True,
silently bypassing HTTP_PROXY / HTTPS_PROXY / ALL_PROXY. Result: every
LINE API call (reply, push, loading, fetch_content, get_bot_user_id)
ignores the system proxy.
Fix: add trust_env=True to all five session constructions. Symmetric
with the wecom and weixin adapters which already set this flag. No
behavior change for users not behind a proxy.