Test MEMORY mode under chunkSizes matrix

The existing chunkSizes parametrized matrix in ReaderTest only routed
through Reader(File, int chunkSize), which hardcodes
FileMode.MEMORY_MAPPED. As a result the chunked file-MEMORY load path
in BufferHolder had no integration coverage — a remainder-chunk-sized-
wrong regression or an EOF-handling change could ship silently.

Add a package-private Reader(File, FileMode, int chunkSize) constructor
and a testMemoryMode(int chunkSize) test that mirrors test(int) but in
MEMORY mode. With chunk sizes 512/2048 against the test DBs (1285 and
2794 bytes), the multi-chunk + remainder branch is now exercised end
to end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: oschwald

src/main/java/com/maxmind/db/Reader.java

@@ -59,8 +59,12 @@ public Reader(File database) throws IOException {
     }
 
     Reader(File database, int chunkSize) throws IOException {
+        this(database, FileMode.MEMORY_MAPPED, chunkSize);
+    }
+
+    Reader(File database, FileMode fileMode, int chunkSize) throws IOException {
         this(
-            new BufferHolder(database, FileMode.MEMORY_MAPPED, chunkSize),
+            new BufferHolder(database, fileMode, chunkSize),
             database.getName(),
             NoCache.getInstance()
         );

src/test/java/com/maxmind/db/ReaderTest.java

@@ -12,6 +12,7 @@
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import com.maxmind.db.Reader.FileMode;
 import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
@@ -83,6 +84,24 @@ public void test(int chunkSize) throws IOException {
         }
     }
 
+    @ParameterizedTest
+    @MethodSource("chunkSizes")
+    public void testMemoryMode(int chunkSize) throws IOException {
+        for (long recordSize : new long[] {24, 28, 32}) {
+            for (int ipVersion : new int[] {4, 6}) {
+                var file = getFile("MaxMind-DB-test-ipv" + ipVersion + "-" + recordSize + ".mmdb");
+                try (var reader = new Reader(file, FileMode.MEMORY, chunkSize)) {
+                    this.testMetadata(reader, ipVersion, recordSize);
+                    if (ipVersion == 4) {
+                        this.testIpV4(reader, file);
+                    } else {
+                        this.testIpV6(reader, file);
+                    }
+                }
+            }
+        }
+    }
+
     static class GetRecordTest {
         InetAddress ip;
         File db;