Discord: failed to create thread when thread already exists for message

[BlakeHung]

Parent: #382

Problem

When a user sends a message that already has a thread created (e.g. from a previous bot interaction or manual thread), openab attempts to create a new thread and fails with:

ERROR openab::discord: failed to create thread: A thread has already been created for this message

The error is not handled gracefully — the bot silently fails to respond.

Steps to reproduce

1. Send a message to the bot in a channel
2. If the bot crashes or restarts, send the same message again (or a message that already has a thread)
3. Observe failed to create thread error in logs
4. Bot does not respond

Expected behavior

If a thread already exists for the message, the bot should reuse the existing thread instead of trying to create a new one.

Environment

• openab 0.7.7 (main HEAD 723d963)
• Backend: claude-agent-acp
• Discord adapter

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