Fix flaky JohnzonJavaxtoJakartaTest and use .reduce in var helper

The test's `2.1.\d+` regex matched `<johnzon.version>2.1.0</johnzon.version>`
before reaching the jakarta.json-api dependency, so when jakarta.json-api
was released as 2.1.3 the expected/actual diverged. Anchor the regex to
the jakarta.json-api artifact.

Also consolidate the `isReassigned` visitor to use `.reduce(...)` directly,
matching the existing idiom in NullCheck and other recipes.

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/lang/var/DeclarationCheck.java

@@ -254,20 +254,20 @@ public static boolean isReassigned(Cursor cursor, J.VariableDeclarations vd) {
         if (method == null) {
             return false;
         }
-        AtomicBoolean reassigned = new AtomicBoolean(false);
-        new JavaIsoVisitor<AtomicBoolean>() {
+        return new JavaIsoVisitor<AtomicBoolean>() {
             @Override
             public J.Assignment visitAssignment(J.Assignment assignment, AtomicBoolean found) {
-                if (!found.get() && assignment.getVariable() instanceof J.Identifier) {
-                    JavaType.Variable fieldType = ((J.Identifier) assignment.getVariable()).getFieldType();
-                    if (variableType.equals(fieldType)) {
-                        found.set(true);
-                    }
+                if (found.get()) {
+                    return assignment;
+                }
+                if (assignment.getVariable() instanceof J.Identifier &&
+                        variableType.equals(((J.Identifier) assignment.getVariable()).getFieldType())) {
+                    found.set(true);
+                    return assignment;
                 }
                 return super.visitAssignment(assignment, found);
             }
-        }.visit(method, reassigned);
-        return reassigned.get();
+        }.reduce(method, new AtomicBoolean(false)).get();
     }
 
     public static J.VariableDeclarations transformToVar(J.VariableDeclarations vd) {

src/test/java/org/openrewrite/java/migrate/jakarta/JohnzonJavaxtoJakartaTest.java

@@ -68,7 +68,8 @@ void migrateJohnzonDependencies() {
                   Matcher version = Pattern.compile("<johnzon.version>([0-9]+\\.[0-9]+\\.[0-9]+)</johnzon.version>")
                     .matcher(actual);
 
-                  Matcher jsonApiVersion = Pattern.compile("2.1.\\d+").matcher(actual);
+                  Matcher jsonApiVersion = Pattern.compile("(?s)<artifactId>jakarta\\.json-api</artifactId>\\s*<version>(2\\.1\\.\\d+)</version>")
+                    .matcher(actual);
                   assertThat(jsonApiVersion.find()).describedAs("Expected jakarta.json-api 2.1.x version in %s", actual).isTrue();
 
                   assertThat(version.find()).isTrue();
@@ -94,7 +95,7 @@ void migrateJohnzonDependencies() {
                         </dependency>
                     </dependencies>
                 </project>
-                """.formatted(version.group(1), jsonApiVersion.group(0));
+                """.formatted(version.group(1), jsonApiVersion.group(1));
               })
             ),
             srcMainJava(