Address PR review comments

- Change LOGGER.error to LOGGER.warn for unknown parameter validation
  Unknown parameters may legitimately occur when users manually correct
  outdated bundled configs, so warning level is more appropriate.

- Use ConcurrentHashMap instead of HashMap in PluginParameterLoader
  Ensures thread safety in multi-threaded Maven builds where the cache
  may be accessed from different worker threads.

Addresses review comments from:
- #389 (review)
- #389 (review)

Author: cowwoc

src/main/java/org/apache/maven/buildcache/xml/CacheConfigImpl.java

@@ -334,7 +334,7 @@ private void validateReconciliationConfig(GoalReconciliation config, Plugin plug
             String propertyName = property.getPropertyName();
 
             if (!goalDef.hasParameter(propertyName)) {
-                LOGGER.error(
+                LOGGER.warn(
                         "Unknown parameter '{}' in reconciliation config for {}:{} version {}. "
                                 + "This may indicate a plugin version mismatch or renamed parameter. "
                                 + "Consider updating parameter definition or removing from reconciliation.",

src/main/java/org/apache/maven/buildcache/xml/PluginParameterLoader.java

@@ -22,8 +22,8 @@
 import javax.xml.parsers.DocumentBuilderFactory;
 
 import java.io.InputStream;
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.apache.maven.buildcache.xml.PluginParameterDefinition.GoalParameterDefinition;
 import org.apache.maven.buildcache.xml.PluginParameterDefinition.ParameterDefinition;
@@ -43,7 +43,7 @@ public class PluginParameterLoader {
     private static final Logger LOGGER = LoggerFactory.getLogger(PluginParameterLoader.class);
     private static final String PARAMETER_DIR = "plugin-parameters/";
 
-    private final Map<String, PluginParameterDefinition> definitions = new HashMap<>();
+    private final Map<String, PluginParameterDefinition> definitions = new ConcurrentHashMap<>();
 
     /**
      * Load parameter definitions for a plugin by artifact ID only (no version matching)