fast-forward spends can be treated as regular spends (#19889)

fast-forward spends can be treated as regular spends if the coin doesn't appear to support the rebasing

Author: arvidn

chia/_tests/core/mempool/test_singleton_fast_forward.py

@@ -663,4 +663,4 @@ async def test_double_spend_ff_spend_no_latest_unspent() -> None:
         status, error = await make_and_send_spend_bundle(sim, sim_client, [singleton_coin_spend], aggsig=sig)
         # It fails validation because it doesn't currently have a latest unspent
         assert status == MempoolInclusionStatus.FAILED
-        assert error == Err.DOUBLE_SPEND
+        assert error == Err.UNKNOWN_UNSPENT

chia/full_node/mempool_manager.py

@@ -640,7 +640,7 @@ async def validate_spend_bundle(
                 assert eligibility_info.ff_puzzle_hash is not None
                 lineage_info = await get_unspent_lineage_info_for_puzzle_hash(eligibility_info.ff_puzzle_hash)
                 if lineage_info is None:
-                    return Err.DOUBLE_SPEND, None, []
+                    mark_as_fast_forward = False
             bundle_coin_spends[coin_id] = BundleCoinSpend(
                 coin_spend=coin_spend,
                 eligible_for_dedup=supports_dedup,