DynamoDb enhanced client: support UpdateExpressions in single-request update

Author: anasatirbasa

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/operations/UpdateItemOperation.java

@@ -270,8 +270,11 @@ public TransactWriteItem generateTransactWriteItem(TableSchema<T> tableSchema, O
     }
 
     /**
-     * Merges UpdateExpressions from POJO attributes, extensions, and request with priority-based resolution.
-     * Attributes referenced in extension expressions are excluded from REMOVE actions to prevent conflicts.
+     * Merges UpdateExpressions from three sources in priority order: POJO attributes (lowest),
+     * extensions (medium), request (highest). Higher priority sources override conflicting actions.
+     * 
+     * <p>Null POJO attributes normally generate REMOVE actions, but are skipped if the same
+     * attribute is referenced in extension/request expressions to avoid DynamoDB conflicts.
      *
      * @param tableMetadata metadata about the table structure
      * @param transformation write modification from extensions containing UpdateExpression

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/update/UpdateExpressionResolver.java

@@ -56,8 +56,8 @@ public static Builder builder() {
      * Merges UpdateExpressions from three sources with priority: item attributes (lowest),
      * extension expressions (medium), request expressions (highest).
      * 
-     * <p><b>Steps:</b> Find attributes in expressions to exclude from removal → generate item SET/REMOVE actions
-     * → merge with extension expressions → merge with request expressions.
+     * <p><b>Steps:</b> Identify attributes used by extensions/requests to prevent REMOVE conflicts → 
+     * create item SET/REMOVE actions → merge extensions (override item) → merge request (override all).
      * 
      * <p><b>Backward compatibility:</b> Without request expressions, behavior is identical to previous versions.
      * <p><b>Exceptions:</b> DynamoDbException may be thrown when the same attribute is updated by multiple sources.