fix(gateway): bypass active-session queueing for slash commands

[canderson26]

Summary

• dispatch recognized gateway slash commands immediately even when a chat already has an active adapter session
• add an end-to-end regression test covering /approve always during an active session

Root cause

Adapter-level session serialization in BasePlatformAdapter.handle_message() queued all follow-up messages behind an active session before gateway command dispatch ran. That meant control-plane commands like /approve and /deny could be queued instead of processed immediately while the agent thread was blocked waiting for approval, leading to 300s timeouts and poisoned follow-up turns.

Why this fix

Gateway slash commands are control-plane operations and should bypass adapter-level active-session queueing. The gateway already has explicit logic to resolve /approve, /deny, /stop, /new, etc. while an agent is running; this change lets those commands actually reach that logic in time.

[canderson26]

Superseded by #5013. Upstream already merged the implementation fix in #4926, so this older implementation PR is being closed in favor of a clean Signal-specific regression-test follow-up.