Fix issue #58: CRITICAL TOCTOU race condition in cache checks

Fixed Time-Of-Check-Time-Of-Use race condition that could cause
NullPointerException in concurrent class loading scenarios.

**Problem:**
Thread A: containsKey(key) → true
Thread B: remove(key) → removes entry
Thread A: get(key) → returns NULL → NPE!

Even though ConcurrentHashMap is thread-safe, two separate operations
(containsKey + get) are not atomic together.

**Files Fixed:**
1. NexusClassSource.java - loadFromMaven() method
2. MavenRepositoryClassSource.java - loadClassData() method
3. MavenNexusClassSource.java - loadClassData() method

**Fix:**
Replaced:
  if (cache.containsKey(key)) {
      return cache.get(key);  // Can return null!
  }

With atomic single operation:
  byte[] data = cache.get(key);
  if (data != null) {
      return data;
  }

This eliminates the race window between check and use.

All 377 tests pass.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Flossy

src/main/java/org/flossware/jclassloader/MavenNexusClassSource.java

@@ -72,8 +72,10 @@ public MavenNexusClassSource(String nexusUrl, String repository, List<MavenArtif
     @Override
     public byte[] loadClassData(String className) throws IOException {
         String cacheKey = className;
-        if (classCache.containsKey(cacheKey)) {
-            return classCache.get(cacheKey);
+        // Atomic get() - avoids TOCTOU race condition with contains() + get()
+        byte[] cachedData = classCache.get(cacheKey);
+        if (cachedData != null) {
+            return cachedData;
         }
 
         String classFileName = ClassNameUtil.toClassFilePath(className);

src/main/java/org/flossware/jclassloader/MavenRepositoryClassSource.java

@@ -67,8 +67,10 @@ public MavenRepositoryClassSource(String repositoryUrl, List<MavenArtifact> arti
     @Override
     public byte[] loadClassData(String className) throws IOException {
         String cacheKey = className;
-        if (classCache.containsKey(cacheKey)) {
-            return classCache.get(cacheKey);
+        // Atomic get() - avoids TOCTOU race condition with contains() + get()
+        byte[] cachedData = classCache.get(cacheKey);
+        if (cachedData != null) {
+            return cachedData;
         }
 
         String classFileName = ClassNameUtil.toClassFilePath(className);

src/main/java/org/flossware/jclassloader/NexusClassSource.java

@@ -116,8 +116,10 @@ private byte[] loadFromMaven(String className) throws IOException {
         }
 
         String cachedKey = packagePath;
-        if (jarCache.containsKey(cachedKey)) {
-            return jarCache.get(cachedKey);
+        // Atomic get() - avoids TOCTOU race condition with contains() + get()
+        byte[] cachedData = jarCache.get(cachedKey);
+        if (cachedData != null) {
+            return cachedData;
         }
 
         String simpleClassName = getSimpleClassName(className);