fix(engine): persist blocks immediately when persistence_threshold=0

Fixes #21093

The `should_persist()` function was using `>` instead of `>=` when
comparing the block count against the persistence threshold. This meant
that with `--engine.persistence-threshold=0`, blocks would not persist
until there was at least 1 block difference (i.e., 2 blocks in memory).

With the corrected `>=` comparison, setting persistence_threshold=0 now
correctly means "persist immediately" (0 blocks allowed in memory without
triggering persistence).

crates/engine/tree/src/tree/mod.rs

@@ -1761,7 +1761,7 @@ where
         }
 
         let min_block = self.persistence_state.last_persisted_block.number;
-        self.state.tree_state.canonical_block_number().saturating_sub(min_block) >
+        self.state.tree_state.canonical_block_number().saturating_sub(min_block) >=
             self.config.persistence_threshold()
     }
 

crates/engine/tree/src/tree/tests.rs

@@ -491,7 +491,7 @@ fn test_tree_persist_block_batch() {
     let chain_spec = MAINNET.clone();
     let mut test_block_builder = TestBlockBuilder::eth().with_chain_spec((*chain_spec).clone());
 
-    // we need more than tree_config.persistence_threshold() +1 blocks to
+    // we need at least tree_config.persistence_threshold() + 1 blocks to
     // trigger the persistence task.
     let blocks: Vec<_> = test_block_builder
         .get_executed_blocks(1..tree_config.persistence_threshold() + 2)
@@ -531,7 +531,7 @@ async fn test_tree_persist_blocks() {
     let chain_spec = MAINNET.clone();
     let mut test_block_builder = TestBlockBuilder::eth().with_chain_spec((*chain_spec).clone());
 
-    // we need more than tree_config.persistence_threshold() +1 blocks to
+    // we need at least tree_config.persistence_threshold() + 1 blocks to
     // trigger the persistence task.
     let blocks: Vec<_> = test_block_builder
         .get_executed_blocks(1..tree_config.persistence_threshold() + 2)