When auto-threading is rate-limited, _auto_create_thread() posted the
'Thread created by Hermes' announcement before confirming the fallback
create_thread() succeeded. On a 429 the announcement was left orphaned
and the agent replied inline, so users saw 'Thread created' with no
thread behind it.
Delete the seed message when the fallback create_thread() fails so the
announcement only survives when a real thread exists.
Adds tests/gateway/test_discord_auto_thread_orphan_seed.py covering the
three paths (fallback-fail deletes seed, fallback-success keeps seed,
direct-success posts no seed).
Fixes#52422
6add84a6a1