[MNG-8572] Fix @PreDestroy ClassNotFoundException caused by premature ClassRealm disposal

The Plexus Disposable.dispose() lifecycle runs before Sisu's @PreDestroy
callbacks. When dispose() called flush(), it disposed ClassRealms before
@PreDestroy methods on beans loaded from those realms could execute,
causing ClassNotFoundException.

Change dispose() to only clear the cache map without disposing realms.
The flush() method (used for explicit cache clearing between builds)
remains unchanged. ClassRealms are disposed when the PlexusContainer
shuts down after all lifecycle callbacks complete.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gnodet

impl/maven-core/src/main/java/org/apache/maven/plugin/DefaultExtensionRealmCache.java

@@ -149,6 +149,6 @@ public void register(MavenProject project, Key key, CacheRecord record) {
 
     @Override
     public void dispose() {
-        flush();
+        cache.clear();
     }
 }

impl/maven-core/src/main/java/org/apache/maven/plugin/DefaultPluginRealmCache.java

@@ -215,6 +215,6 @@ public void register(MavenProject project, Key key, CacheRecord record) {
 
     @Override
     public void dispose() {
-        flush();
+        cache.clear();
     }
 }

impl/maven-core/src/main/java/org/apache/maven/project/DefaultProjectRealmCache.java

@@ -125,6 +125,6 @@ public void register(MavenProject project, Key key, CacheRecord record) {
 
     @Override
     public void dispose() {
-        flush();
+        cache.clear();
     }
 }