4.x: Codegen Unit Testing (helidon-io#11283)

* Codegen Unit Testing

Add a new module helidon-codegen-testing to facilitate writing using tests for code generators.
The module contains a unit test that demonstrates how to use it.

* Review feedback

* Updates to builder to align with Helidon builder style.

* Add checks for non-null.

---------

Co-authored-by: Tomas Langer <tomas.langer@oracle.com>

Author: romain-grecourt

codegen/pom.xml

@@ -44,6 +44,7 @@
         <module>apt</module>
         <module>scan</module>
         <module>compiler</module>
+        <module>testing</module>
     </modules>
 
     <build>

codegen/testing/pom.xml

@@ -0,0 +1,64 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    Copyright (c) 2026 Oracle and/or its affiliates.
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+-->
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
+        xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>io.helidon.codegen</groupId>
+        <artifactId>helidon-codegen-project</artifactId>
+        <version>4.4.0-SNAPSHOT</version>
+    </parent>
+    <artifactId>helidon-codegen-testing</artifactId>
+    <name>Helidon Codegen Testing</name>
+    <description>
+        Tools for testing codegen extensions
+    </description>
+
+    <properties>
+        <helidon.services.skip>true</helidon.services.skip>
+        <maven.deploy.skip>true</maven.deploy.skip>
+        <maven.sources.skip>true</maven.sources.skip>
+        <maven.javadoc.skip>true</maven.javadoc.skip>
+        <spotbugs.skip>true</spotbugs.skip>
+        <dependency-check.skip>true</dependency-check.skip>
+        <checkstyle.skip>true</checkstyle.skip>
+        <dependency-plugin-check-dependencies.skip>true</dependency-plugin-check-dependencies.skip>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>io.helidon.codegen</groupId>
+            <artifactId>helidon-codegen</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.hamcrest</groupId>
+            <artifactId>hamcrest-all</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.jetbrains</groupId>
+            <artifactId>annotations</artifactId>
+            <optional>true</optional>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

codegen/testing/src/main/java/io/helidon/codegen/testing/CodegenMatchers.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2026 Oracle and/or its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.helidon.codegen.testing;
+
+import java.util.regex.Pattern;
+import java.util.stream.Collectors;
+
+import org.hamcrest.Description;
+import org.hamcrest.Matcher;
+import org.hamcrest.TypeSafeMatcher;
+import org.intellij.lang.annotations.Language;
+
+/**
+ * Hamcrest matchers for codegen tests.
+ * <p>
+ * <b>This is NOT part of any supported API.
+ * If you write code that depends on this, you do so at your own risk.
+ * This code and its internal interfaces are subject to change or deletion without notice.</b>
+ * </p>
+ */
+public final class CodegenMatchers {
+
+    private CodegenMatchers() {
+    }
+
+    /**
+     * Regex match that treats {@code //...} as multiline wildcards.
+     *
+     * @param expected expected string
+     * @return matcher
+     */
+    public static Matcher<String> matches(@Language("java") String expected) {
+        return new RegexMatcher(expected);
+    }
+
+    private static final class RegexMatcher extends TypeSafeMatcher<String> {
+
+        private final String regex;
+
+        RegexMatcher(String expected) {
+            var regex = expected.lines().map(it -> {
+                if (it.trim().startsWith("//...")) {
+                    return ".*";
+                } else {
+                    return "\\Q" + it + "\\E";
+                }
+            }).collect(Collectors.joining("\n"));
+            this.regex = expected.endsWith("\n") ? regex + "\n" : regex;
+        }
+
+        @Override
+        public void describeTo(Description description) {
+            description.appendText(regex);
+        }
+
+        @Override
+        protected boolean matchesSafely(String item) {
+            var pattern = Pattern.compile(regex, Pattern.MULTILINE | Pattern.DOTALL);
+            return pattern.matcher(item).matches();
+        }
+    }
+}