test(client-v2): assert enum-null error names column and type; clarify changelog

Address review feedback on #2932:
- Strengthen testNullIntoNonNullableEnumThrowsIllegalArgument to assert the
  error message names the offending column and includes the enum type string,
  not just the generic prefix, so a future refactor that drops either detail
  fails the test. Use a distinctive `bs_flag` column name (matching the issue
  repro) so the column-name assertion is unambiguous.
- Reword CHANGELOG: serializeEnumData had no null guard (there was no prior
  null check), avoiding the implication of a pre-existing check.

Author: polyglotAI-bot

CHANGELOG.md

@@ -5,11 +5,11 @@
 ### Bug Fixes 
 
 - **[client-v2]** Fixed a `NullPointerException` when serializing a `null` value into a non-nullable
-  `Enum8`/`Enum16` column. `SerializerUtils.serializeEnumData` dereferenced the value before the `null`
-  check, so a `null` in a non-nullable enum column failed the RowBinary insert path with a confusing NPE
-  instead of a clear error. It now throws `IllegalArgumentException` naming the column, consistent with the
-  existing `IllegalArgumentException` for other unsupported enum values. Nullable enum columns are unaffected.
-  (https://github.com/ClickHouse/clickhouse-java/issues/2931)
+  `Enum8`/`Enum16` column. `SerializerUtils.serializeEnumData` had no `null` guard, so a `null` in a
+  non-nullable enum column reached `value.getClass()` and failed the RowBinary insert path with a confusing
+  NPE instead of a clear error. It now throws `IllegalArgumentException` naming the column, consistent with
+  the existing `IllegalArgumentException` for other unsupported enum values. Nullable enum columns are
+  unaffected. (https://github.com/ClickHouse/clickhouse-java/issues/2931)
 - **[client-v2]** Fixed POJO insert error classification so transport write failures such as java.net.SocketException:
   Broken pipe (Write failed) are now surfaced as transfer/network errors instead of being wrapped as
   DataSerializationException. This only changes the exception type reported for request-body transport failures during

client-v2/src/test/java/com/clickhouse/client/api/data_formats/internal/SerializerUtilsTest.java

@@ -145,12 +145,17 @@ public void testGeometrySerializationRejectsMalformedList() {
 
     @Test(dataProvider = "nonNullableEnumTypes")
     public void testNullIntoNonNullableEnumThrowsIllegalArgument(String typeName) {
-        ClickHouseColumn column = ClickHouseColumn.of("v", typeName);
+        ClickHouseColumn column = ClickHouseColumn.of("bs_flag", typeName);
 
         IllegalArgumentException ex = Assert.expectThrows(IllegalArgumentException.class,
                 () -> SerializerUtils.serializeData(new ByteArrayOutputStream(), null, column));
-        Assert.assertTrue(ex.getMessage().contains("Cannot insert null into non-nullable column"),
-                "Unexpected message: " + ex.getMessage());
+        String message = ex.getMessage();
+        Assert.assertTrue(message.contains("Cannot insert null into non-nullable column"),
+                "Unexpected message: " + message);
+        Assert.assertTrue(message.contains("bs_flag"),
+                "Message should name the offending column: " + message);
+        Assert.assertTrue(message.contains(typeName),
+                "Message should include the enum type: " + message);
     }
 
     @DataProvider(name = "nonNullableEnumTypes")