address gemini review changes

Author: nidhiii-27

java-storage/google-cloud-storage/src/main/java/com/google/cloud/storage/ApiaryUnbufferedReadableByteChannel.java

@@ -143,7 +143,7 @@ public long read(ByteBuffer[] dsts, int offset, int length) throws IOException {
             Crc32cValue<?> expected = Crc32cValue.of(expectedVal, 0);
             Crc32cValue.Crc32cLengthKnown actual = Crc32cValue.of(calculatedCrc32c, 0);
 
-            Hasher.defaultHasher().validate(expected, actual);
+            hasher.validate(expected, actual);
           }
         } else {
           totalRead += read;
@@ -238,7 +238,7 @@ private ScatteringByteChannel open() {
 
       boolean isHasherEnabled = !(hasher instanceof Hasher.NoOpHasher);
       boolean isFullObjectDownload = (request.getByteRangeSpec().getHttpRangeHeader() == null);
-      if (isHasherEnabled && isFullObjectDownload) {
+      if (isHasherEnabled && isFullObjectDownload && expectedCrc32cBase64 != null) {
         this.hashingInputStream = new HashingInputStream(Hashing.crc32c(), content);
         content = this.hashingInputStream;
       }

java-storage/google-cloud-storage/src/test/java/com/google/cloud/storage/ApiaryUnbufferedReadableByteChannelTest.java

@@ -121,16 +121,15 @@ public void testRead_mismatchedCrc32cValidation_throwsChecksumMismatch() throws
             Hasher.defaultHasher()); ) {
 
       ByteBuffer buf = ByteBuffer.allocate(4096);
-      IOException expected =
+      Hasher.ChecksumMismatchException expected =
           assertThrows(
-              IOException.class,
+              Hasher.ChecksumMismatchException.class,
               () -> {
                 while (channel.read(new ByteBuffer[] {buf}, 0, 1) != -1) {
                   buf.clear();
                 }
               });
 
-      assertTrue(expected instanceof Hasher.ChecksumMismatchException);
       assertTrue(expected.getMessage().contains("Mismatch checksum value"));
     }
   }