Update configuration description and add regression tests

Document the \${...} escape syntax in the ChangePluginConfiguration
@option description so users know how to pass literal Maven property
references.

Add regression tests to ensure existing behavior is preserved:
- Unresolved placeholders are left as-is
- Normal resolution still works
- Backslashes not before a placeholder prefix are preserved

Author: MBoegers

rewrite-core/src/test/java/org/openrewrite/internal/PropertyPlaceholderHelperTest.java

@@ -112,6 +112,31 @@ void mixedEscapedAndResolvedPlaceholders() {
         assertThat(s).isEqualTo("hello ${java.version}");
     }
 
+    @Test
+    void unresolvedPlaceholderLeftAsIs() {
+        var helper = new PropertyPlaceholderHelper("${", "}", null);
+        var s = helper.replacePlaceholders("${unresolved}", k -> null);
+        assertThat(s).isEqualTo("${unresolved}");
+    }
+
+    @Test
+    void normalResolutionStillWorks() {
+        var helper = new PropertyPlaceholderHelper("${", "}", null);
+        var s = helper.replacePlaceholders("${greeting} ${name}", k -> switch (k) {
+            case "greeting" -> "hello";
+            case "name" -> "world";
+            default -> null;
+        });
+        assertThat(s).isEqualTo("hello world");
+    }
+
+    @Test
+    void backslashNotBeforePrefixIsPreserved() {
+        var helper = new PropertyPlaceholderHelper("${", "}", null);
+        var s = helper.replacePlaceholders("path\\to\\file", k -> null);
+        assertThat(s).isEqualTo("path\\to\\file");
+    }
+
     @Test
     void withValueSeparatorAndNullReplacement() {
         var helper = new PropertyPlaceholderHelper("%%{", "}", ",");

rewrite-maven/src/main/java/org/openrewrite/maven/ChangePluginConfiguration.java

@@ -50,7 +50,10 @@ public class ChangePluginConfiguration extends Recipe {
     @Option(displayName = "Configuration",
             description = "Plugin configuration provided as raw XML overriding any existing configuration. " +
                           "Configuration inside `<executions>` blocks will not be altered. " +
-                          "Supplying `null` will remove any existing configuration.",
+                          "Supplying `null` will remove any existing configuration. " +
+                          "To include a literal `${...}` property reference in the configuration " +
+                          "(e.g. a Maven property like `${java.version}`), escape it as `\\${...}` " +
+                          "in your recipe YAML to prevent it from being resolved as a recipe placeholder.",
             example = "<foo>bar</foo>",
             required = false)
     @Nullable