CI: fix clippy collapsible_match on stable 1.96

rustc 1.96 expanded clippy::collapsible_match to flag a match arm whose
sole body is an `if`, breaking the `-D warnings` gate (CI runs
dtolnay/rust-toolchain@stable = 1.96.1; local was 1.90). Move the
pacs.002 `find_first(...).is_none()` check into a match guard as clippy
suggests. No behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rustyconover

crates/iso20022-core/src/validate.rs

@@ -177,10 +177,8 @@ fn validate_mx(xml: &str, id: &str, errors: &mut Vec<String>) {
                 check_amount_node(Some(amt), "Amt", errors);
             }
         }
-        "pacs.002" => {
-            if root.find_first("FIToFIPmtStsRpt").is_none() {
-                errors.push("MX020: pacs.002 missing FIToFIPmtStsRpt".into());
-            }
+        "pacs.002" if root.find_first("FIToFIPmtStsRpt").is_none() => {
+            errors.push("MX020: pacs.002 missing FIToFIPmtStsRpt".into());
         }
         _ => {}
     }