fix(outbound): keep reply_to on every chunk when threading

[Ydz0616]

Problem

In a Lark thread/topic chat, the tail of a long threaded reply leaks out of the thread into the main chat.

OutboundSender._send_text splits a message over text_chunk_limit into multiple bodies but applies reply_to only to the first chunk (reply_to if idx == 0 else None). The comment assumes "fresh messages render in the original chat" — true for a normal reply, but in a thread/topic chat a message sent without a reply parent renders in the main chat, not the thread. So chunks after the first leak out of the topic.

Fix

Keep reply_to on every chunk when reply_in_thread is set:

effective_reply_to = reply_to if (idx == 0 or reply_in_thread) else None

Each chunk replies to the same root with reply_in_thread=True, so all pieces stay in the thread. Non-thread replies are unchanged (still first-chunk-only quote).

Reproduction

Topic-mode group, send a >3500-char markdown message with SendOpts(reply_to=<root>, reply_in_thread=True):

• before: first chunk threaded, remaining chunks in the main chat;
• after: all chunks in the thread.

Closes #5.

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