fix: ensure session for slash replies

[yqmaphy]

Summary

• lazily create a session before wrapping slash command replies in the WeChat bridge when the conversation has not started a session yet
• keep /help behavior unchanged so help output stays lightweight and does not create a session
• closes Fix missing session context for WeChat slash replies #224

Testing

• not run locally; the standard pre-push hook failed in this environment while running bun turbo typecheck with an OS-level I/O error