Fix RefineSwitchCases generating invalid guard on enum constant labels (#1124)

`when` guards are only valid on pattern case labels, but the recipe
also matched constant labels (e.g. enum constants) when the `if`
condition referenced no label-bound variables, producing uncompilable
code with a duplicate case. Require the case label to bind pattern
variables before refining.

Fixes #1123

Author: timtebeek

src/main/java/org/openrewrite/java/migrate/lang/RefineSwitchCases.java

@@ -66,8 +66,11 @@ public J.Switch visitSwitch(J.Switch sw, ExecutionContext ctx) {
                                 List<Statement> caseStatements = ((J.Block) case_.getBody()).getStatements();
                                 if (caseStatements.size() == 1 && caseStatements.get(0) instanceof J.If) {
                                     J.If if_ = (J.If) caseStatements.get(0);
-                                    if (extractLabelVariables(case_)
-                                            .containsAll(extractConditionVariables(if_.getIfCondition().getTree()))) {
+                                    // Guards (`when`) are only valid on pattern case labels, not on
+                                    // constant labels such as enum constants, so require bound variables
+                                    Set<String> labelVariables = extractLabelVariables(case_);
+                                    if (!labelVariables.isEmpty() &&
+                                            labelVariables.containsAll(extractConditionVariables(if_.getIfCondition().getTree()))) {
                                         // Replace case with multiple cases
                                         return createGuardedCases(case_, if_);
                                     }

src/test/java/org/openrewrite/java/migrate/lang/RefineSwitchCasesTest.java

@@ -293,6 +293,42 @@ else if (i >= 0 && i < 5)
             );
         }
 
+        @Test
+        void enumConstantLabelWithGuardlessCondition() {
+            rewriteRun(
+              //language=java
+              java(
+                """
+                  class Reproducer {
+                      enum EnumOptions {
+                          OPTION_ONE,
+                          OPTION_TWO
+                      }
+
+                      boolean checkMethod() {
+                          return true;
+                      }
+
+                      void handleOptionOne() {
+                      }
+
+                      void buildMenu(EnumOptions item) {
+                          switch (item) {
+                              case OPTION_ONE -> {
+                                  if (checkMethod()) {
+                                      handleOptionOne();
+                                  }
+                              }
+                              case OPTION_TWO -> {
+                              }
+                          }
+                      }
+                  }
+                  """
+              )
+            );
+        }
+
         @Test
         void notFormattedWhenNotChanged() {
             rewriteRun(