fix: unify context_window_size fallback defaults across all channels

[yaojin3616]

Background

PR #232 fixed context_window_size persistence and usage in websocket.py and feishu.py. However, multiple other channel integrations still use inconsistent fallback default values.

Problem

When agent.context_window_size is None or 0, different channels fall back to different values:

File	Current Fallback	Expected
websocket.py:462	or 100	✅
feishu.py:398	or 100	✅
feishu.py:932	or 100	✅
feishu.py:1235	or 100	✅
discord_gateway.py:173	or 20	❌
wecom_stream.py:292	or 20	❌
wecom.py:473	or 20	❌
slack.py:232	or 20	❌
discord_bot.py:294	or 20	❌
teams.py:482	or 20	❌
dingtalk.py:169	or 20	❌

Suggested Fix

1. Define a single constant (e.g. DEFAULT_CONTEXT_WINDOW_SIZE = 100) in a shared location like app/models/agent.py or a config module.
2. Replace all or 20 / or 100 fallbacks with the shared constant.
3. This ensures consistent behavior across web chat, Feishu, Slack, Discord, WeCom, DingTalk, and Teams channels.

Related

• Merged: fix(agent): make context_window_size persist and take effect in context loading #232