feat: add .mvn/extensions.xml to Maven cache key pattern (#1041)

* Initial plan

* feat: add .mvn/extensions.xml to Maven cache key pattern

Closes #990

Maven build extensions declared in `.mvn/extensions.xml` can introduce
additional plugin dependencies (e.g. lifecycle participants, custom
packaging types). Including this file in the cache key hash ensures that
changes to extensions — which affect what plugin JARs Maven downloads —
properly invalidate the cache, preventing stale caches from missing
newly-required plugin dependencies.

Changes:
- src/cache.ts: add `**/.mvn/extensions.xml` to Maven pattern array
- __tests__/cache.test.ts: update pattern expectations; add new test
- README.md: document the new file in the Maven cache key hash list

* test: update maven cache error test name for extensions.xml

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Bruno Borges <brborges@microsoft.com>

dist/cleanup/index.js

@@ -51973,7 +51973,11 @@ const supportedPackageManager = [
             (0, path_1.join)(os_1.default.homedir(), '.m2', 'wrapper', 'dists')
         ],
         // https://github.com/actions/cache/blob/0638051e9af2c23d10bb70fa9beffcad6cff9ce3/examples.md#java---maven
-        pattern: ['**/pom.xml', '**/.mvn/wrapper/maven-wrapper.properties']
+        pattern: [
+            '**/pom.xml',
+            '**/.mvn/wrapper/maven-wrapper.properties',
+            '**/.mvn/extensions.xml'
+        ]
     },
     {
         id: 'gradle',

dist/setup/index.js

@@ -77838,7 +77838,11 @@ const supportedPackageManager = [
             (0, path_1.join)(os_1.default.homedir(), '.m2', 'wrapper', 'dists')
         ],
         // https://github.com/actions/cache/blob/0638051e9af2c23d10bb70fa9beffcad6cff9ce3/examples.md#java---maven
-        pattern: ['**/pom.xml', '**/.mvn/wrapper/maven-wrapper.properties']
+        pattern: [
+            '**/pom.xml',
+            '**/.mvn/wrapper/maven-wrapper.properties',
+            '**/.mvn/extensions.xml'
+        ]
     },
     {
         id: 'gradle',