Fix spurious debug_assert in UTXO gossip dedup check

[tnull]

Fixes #4439.

check_replace_previous_entry hit debug_assert!(false) when channel_announce was None on a still-live UtxoMessages. The comment claimed this was unreachable because channel_announce is set under the same lock as the channel map entry. However, there is a legitimate race:

1. A channel announcement arrives, an async UTXO lookup starts, and pending_channels[scid] is set with a Weak to the UtxoMessages.
2. The lookup resolves. resolve_single_future takes both channel_announce and complete via .take(), but the Arc<Mutex<UtxoMessages>> is still alive on the stack of check_resolved_futures.
3. A duplicate announcement for the same SCID arrives during this window. check_replace_previous_entry upgrades the Weak, finds channel_announce is None, and hits the assert.

Replace the unconditional debug_assert!(false) with a targeted check that complete has also been taken (confirming the future resolved), which would catch a genuinely unexpected state where channel_announce is None but complete is still pending.

Co-Authored-By: HAL 9000

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