Issue: Improve visibility when Weixin session expires

Title

Improve visibility when Weixin session expires

Problem

When the Weixin adapter session expires, users may only observe that replies stop arriving. The logs contain messages such as Session expired and send timeout errors, but Web UI/gateway health surfaces do not clearly tell the user that the Weixin account/session needs attention.

Observed Evidence

Recent local gateway logs contained repeated entries like:

• gateway.platforms.weixin: [Weixin] Session expired; pausing for 10 minutes
• sendmessage error ... session timeout

The gateway and API server can otherwise be running, so a generic stopped or no reply symptom is misleading.

Proposal

Expose Weixin session-expired state as an actionable gateway/platform diagnostic.

Potential outputs:

• Gateway detailed health includes platform status: weixin=session_expired.
• Web UI Gateway/Channels page shows: Weixin session expired; reconnect or refresh credentials.
• A doctor/smoke command reports the same warning and points to gateway.log.

Non-Goals

• Do not automatically re-login.
• Do not loop aggressive retries.
• Do not mutate credentials or session files.
• Do not expose secrets or raw tokens in diagnostics.

Acceptance Criteria

• A user can distinguish gateway is down from gateway is running but Weixin session expired.
• The diagnostic includes a clear next action.
• Existing adapters that do not support structured platform diagnostics are unaffected.

Risk

Medium. The implementation depends on how platform adapters currently report runtime state, but the user-facing behavior is important for troubleshooting.