Fix shared CST state for injected method (#411)

This method fixes a subtle bug that occurs when the CST for a pre-cached
node is shared between codemod uses.

framework/codemodder-base/src/main/java/io/codemodder/remediation/headerinjection/DefaultHeaderInjectionRemediator.java

@@ -25,19 +25,17 @@
 final class DefaultHeaderInjectionRemediator implements HeaderInjectionRemediator {
 
   private static final Set<String> setHeaderNames = Set.of("setHeader", "addHeader");
-  private final MethodDeclaration fixMethod;
 
   private static final String validatorMethodName = "stripNewlines";
+  private final String fixMethodCode;
 
   DefaultHeaderInjectionRemediator() {
-    String fixMethodCode =
+    this.fixMethodCode =
         """
             private static String stripNewlines(final String s) {
               return s.replaceAll("[\\n\\r]", "");
             }
             """;
-
-    this.fixMethod = StaticJavaParser.parseMethodDeclaration(fixMethodCode);
   }
 
   @Override
@@ -89,6 +87,11 @@ public <T> CodemodFileScanningResult remediateAll(
                             && md.getParameters().get(0).getTypeAsString().equals("String"));
 
         if (!alreadyHasResourceValidationCallPresent) {
+          // one might be tempted to cache this result, but then it will be a shared resource with
+          // shared CST metadata and cause bugs
+          MethodDeclaration fixMethod = StaticJavaParser.parseMethodDeclaration(fixMethodCode);
+
+          // add the method to the class
           parentClass.addMember(fixMethod);
         }
       }