fix: send file attachments before final stream reply in WeCom WebSocket channel

[highland0971]

PR 3: Fix WeCom File Attachment Timing — Send Before Final Stream Reply

Problem

On the WeCom (WeChat Work / 企业微信) channel, file attachments sent using
send_file after a final stream reply (reply_stream(is_final=True)) are
silently ignored by WeCom. The WeCom WebSocket protocol closes the stream
context after the final reply, so any subsequent send_file calls fail without
clear error feedback.

This means users receive the AI's text response but never receive the
associated file artifacts generated by the agent.

Root Cause

The WeComChannel.on_send() method was structured as:

1. Call self.send(msg) — which sends the text reply (including is_final=True)
2. Iterate over msg.attachments and call self.send_file()
3. Clear WebSocket context

When step 1 sends is_final=True, WeCom WebSocket closes the message context,
and step 2's file uploads are ignored.

Additionally, the send_file method itself also had an early return for
is_final=False messages, which blocked non-final attachments from being sent.

Fix

wecom.py — Reorder send sequence

• File attachments are now sent BEFORE the final stream reply when
  msg.is_final=True.
• For non-final messages, attachments continue to be sent after the reply
  (since the stream context remains open).
• The WebSocket context clearing (_clear_ws_context) remains at the end.

New send order for final messages:

1. Send file attachments (send_file)
2. Send text reply (send / reply_stream(is_final=True))
3. Clear WebSocket context

Testing

• Verified in production: file artifacts now arrive alongside the AI's
  final response on WeCom.
• No regression on non-WeCom channels (the fix is WeCom-specific).

Files Changed

• backend/app/channels/wecom.py