fix(rpc/test): clippy map_or_else + redundant closure on merkle_root construction

The smoke fixture in commit 6f93ded uses bitcoin::merkle_tree::calculate_root
to derive the merkle root from a single-tx block. The map(...).unwrap_or_else()
chain triggered clippy::map_unwrap_or, and the closure was a redundant wrapper
for TxMerkleNode::from_raw_hash.

Replace with map_or_else passing the two associated functions directly.

Op: correct
Restores: rule:clippy_workspace_clean

Author: metaphorics

crates/rpc/tests/handler_smoke.rs

@@ -350,8 +350,10 @@ impl Fixture {
                 merkle_root: bitcoin::merkle_tree::calculate_root(std::iter::once(
                     tx.compute_txid().to_raw_hash(),
                 ))
-                .map(|root| bitcoin::TxMerkleNode::from_raw_hash(root))
-                .unwrap_or_else(bitcoin::TxMerkleNode::all_zeros),
+                .map_or_else(
+                    bitcoin::TxMerkleNode::all_zeros,
+                    bitcoin::TxMerkleNode::from_raw_hash,
+                ),
                 time: 1_231_006_505,
                 bits: bitcoin::CompactTarget::from_consensus(0x1d00_ffff),
                 nonce: 0,