Address review: clarify serializeNestedData SAF exception, fix CHANGELOG code spans

- SerializerUtils: expand the top-level SimpleAggregateFunction case comment
  and serializeNestedData's Javadoc to note the deliberate exception to the
  "nested elements only" contract (the SAF wrapper is non-nullable so
  writeValuePreamble writes no marker, but a Nullable underlying still needs one).
- CHANGELOG: reflow the SimpleAggregateFunction entry so no inline-code span
  wraps across a line break (renders correctly in Markdown), matching the
  file's other entries.

Both changes are documentation-only; no behavior change.

Author: polyglotAI-bot

CHANGELOG.md

@@ -4,11 +4,12 @@
 
 ### Bug Fixes 
 
-- **[client-v2]** Fixed the `RowBinary` writer throwing `UnsupportedOperationException: Unsupported data type:
-  SimpleAggregateFunction` when inserting into a `SimpleAggregateFunction(func, T)` column (the reader already
-  supported these columns). The value is now serialized identically to its underlying type `T`, writing the
-  `Nullable` null-marker byte when the underlying type is nullable (e.g. `SimpleAggregateFunction(anyLast,
-  Nullable(String))`), mirroring the read path. (https://github.com/ClickHouse/clickhouse-java/issues/2477)
+- **[client-v2]** Fixed the `RowBinary` writer throwing
+  `UnsupportedOperationException: Unsupported data type: SimpleAggregateFunction` when inserting into a
+  `SimpleAggregateFunction(func, T)` column (the reader already supported these columns). The value is now
+  serialized identically to its underlying type `T`, writing the `Nullable` null-marker byte when the
+  underlying type is nullable (e.g. `SimpleAggregateFunction(anyLast, Nullable(String))`), mirroring the
+  read path. (https://github.com/ClickHouse/clickhouse-java/issues/2477)
 
 - **[client-v2]** Fixed binary array decoding for nullable element types so `Array(Nullable(Float64))` and similar columns now return boxed arrays such as `Double[]` instead of `Object[]`. This keeps null-supporting arrays aligned with their element type while preserving the existing `Object[]` fallback for Variant/Dynamic/Geometry arrays. (https://github.com/ClickHouse/clickhouse-java/issues/2846)
 

client-v2/src/main/java/com/clickhouse/client/api/data_formats/internal/SerializerUtils.java

@@ -74,8 +74,11 @@ public static void serializeData(OutputStream stream, Object value, ClickHouseCo
                 break;
             case SimpleAggregateFunction:
                 // A SimpleAggregateFunction(func, T) value serializes identically to its underlying
-                // type T; delegate through serializeNestedData so a Nullable underlying still gets
-                // its RowBinary null-marker byte, mirroring BinaryStreamReader's read path.
+                // type T. This is a deliberate exception to serializeNestedData's "nested elements
+                // only" contract: the SAF wrapper is itself non-nullable, so writeValuePreamble emits
+                // no null-marker for a top-level SAF column, yet a Nullable underlying still needs one.
+                // serializeNestedData writes that marker iff the underlying is nullable, mirroring
+                // BinaryStreamReader's read path.
                 serializeNestedData(stream, value, column.getNestedColumns().get(0));
                 break;
             case AggregateFunction:
@@ -123,7 +126,10 @@ public static void serializeData(OutputStream stream, Object value, ClickHouseCo
      * {@code 0x01} when null), as the server expects for {@code Nullable} sub-columns in
      * {@code RowBinary}. For a top-level column this marker is instead written by
      * {@link com.clickhouse.client.api.data_formats.RowBinaryFormatSerializer#writeValuePreamble},
-     * so this helper must only be used for nested elements.
+     * so this helper is normally used only for nested elements. The one deliberate exception is a
+     * top-level {@code SimpleAggregateFunction} column: its wrapper is itself non-nullable (so
+     * {@code writeValuePreamble} writes no marker), but its underlying type may be {@code Nullable}
+     * and still needs the marker, which this helper supplies.
      */
     private static void serializeNestedData(OutputStream stream, Object value, ClickHouseColumn column) throws IOException {
         if (column.isNullable()) {