fix(messages): don't export whole history when unread_count is 0 - #307

What changed

export_unread_messages is annotated read-only, but with unread_count == 0 its batching loop fell back to a limit of 100 per page and the unread-based stop condition never fired — so it exported the entire chat history to disk. The loop is now gated on while unread_count:; a zero-unread chat still gets an entry with unread_count_at_export: 0 and an empty message list.

Before

Chat with 350 messages in history, unread_count = 0 (fully read):

export_unread_messages(chat_ids=[123])
→ {"status": "ok", "chats_processed": 1, "messages_exported": 350, ...}
out.json: all 350 messages written to disk

The read-only tool dumped the whole history — nothing is unread, everything was exported.

After

Same chat, same call:

export_unread_messages(chat_ids=[123])
→ {"status": "ok", "chats_processed": 1, "messages_exported": 0, ...}
out.json: "chats": {"123": {"unread_count_at_export": 0, "messages_exported": 0, "messages": []}}

No get_messages calls are made; only truly unread messages are exported.

Tests

• tests/test_export_unread_messages.py: new test_export_skips_chat_with_zero_unread — 350 messages, unread_count=0 → no get_messages calls, messages_exported: 0.
• Full suite: uv run pytest -q → 1196 passed.

Fixes #296

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