fix: send SyncStatus/ackDiff after processing buyer messages to clear unread indicators

[octo-patch]

Fixes #89

Problem

When the bot receives a message from a buyer and sends an auto-reply, it was not sending a read acknowledgment (SyncStatus/ackDiff) back to the Xianyu server. As a result, the seller's Xianyu interface continued to show those buyer messages as unread, even though the bot had already processed and replied to them. This creates a confusing experience where the seller sees a non-zero unread count despite the bot handling all messages.

Solution

Add a send_read_ack method that sends a SyncStatus/ackDiff WebSocket message to the server after processing each buyer message. This follows the same protocol used in init() to acknowledge sync state.

The pts (point-in-time state) value is extracted from the incoming sync package (sync_data.get("pts", ...)), with a fallback to create_time * 1000 (converting the message timestamp from milliseconds to microseconds) if the sync package does not include a pts field.

send_read_ack is called in two places:

1. After sending an auto-reply to the buyer
2. When the bot decides not to reply (bot_reply == "-") — the message is still "seen" by the bot and should be marked as read

Testing

• The fix is additive: it sends an additional WebSocket message that mirrors the sync ack already sent during init().
• Error handling wraps the send so that a failure to send the ack does not interrupt message processing.
• No new dependencies or schema changes introduced.