Add unit tests for input stream helper

src/test/java/de/bund/digitalservice/a2j/security/CachedBodyServletInputStreamTest.java

@@ -0,0 +1,63 @@
+package de.bund.digitalservice.a2j.security;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import jakarta.servlet.ReadListener;
+import java.io.IOException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class CachedBodyServletInputStreamTest {
+
+  private CachedBodyServletInputStream inputStream;
+  private byte[] data;
+
+  @BeforeEach
+  void setup() {
+    data = "test".getBytes();
+    inputStream = new CachedBodyServletInputStream(data);
+  }
+
+  @Test
+  void testRead() throws IOException {
+    for (byte b : data) {
+      assertEquals(b, inputStream.read());
+    }
+    assertEquals(-1, inputStream.read());
+  }
+
+  @Test
+  void testIsFinished() throws IOException {
+    assertFalse(inputStream.isFinished());
+
+    while (inputStream.read() != -1) {}
+
+    assertTrue(inputStream.isFinished());
+  }
+
+  @Test
+  void testIsReady() {
+    assertTrue(inputStream.isReady());
+  }
+
+  @Test
+  void testSetReadListener() {
+    assertThrows(
+        UnsupportedOperationException.class,
+        () ->
+            inputStream.setReadListener(
+                new ReadListener() {
+                  @Override
+                  public void onDataAvailable() throws IOException {}
+
+                  @Override
+                  public void onAllDataRead() throws IOException {}
+
+                  @Override
+                  public void onError(Throwable throwable) {}
+                }));
+  }
+}