[fluss-client] Support Complex Data Types on the Java API (NestedRow/ROW)

[XuQianJin-Stars]

Purpose

Linked issue: close #2805

This pull request extends the Java Typed API to support the ROW data type (nested POJO) in POJO ↔ InternalRow conversion, completing the complex type coverage alongside the already supported ARRAY and MAP types.

Brief change log

• ConverterCommons.java: Added ROW type validation in validateCompatibility() — ensures the POJO field for a ROW type is not a primitive, array, Collection, or Map. Detailed field-level validation is deferred to the nested converter.
• PojoToRowConverter.java: Added case ROW: in createFieldConverter() — recursively creates a nested PojoToRowConverter to convert a nested POJO field into a GenericRow.
• RowToPojoConverter.java: Added case ROW: in createRowReader() — recursively creates a nested RowToPojoConverter to convert an InternalRow back into a nested POJO.
• PojoTypeToFlussTypeConverter.java: Added case ROW: in convertElementValue() — supports serializing nested ROW elements within ARRAY and MAP types.
• FlussArrayToPojoArray.java: Added case ROW: in buildElementConverter() — supports deserializing ROW elements within arrays. Falls back to returning raw InternalRow when the target POJO type is Object.class (due to Java type erasure in MAP values).
• java-client.md: Added ROW → Nested POJO entry to the type mapping table and a new "Nested POJOs (ROW Type)" documentation section with usage examples and a note about the Java type erasure limitation for MAP<K, ROW>.

Tests

• NestedRowConverterTest#testSimpleNestedRowRoundTrip — basic POJO with a nested ROW field, roundtrip serialization/deserialization
• NestedRowConverterTest#testNullNestedRow — null nested POJO is correctly handled
• NestedRowConverterTest#testDeeplyNestedRowRoundTrip — two-level deep nesting (POJO → ROW → ROW)
• NestedRowConverterTest#testNestedRowWithArrayFieldRoundTrip — nested POJO containing an ARRAY<INT> field
• NestedRowConverterTest#testNestedRowWithMapFieldRoundTrip — nested POJO containing a MAP<STRING, INT> field
• NestedRowConverterTest#testArrayOfNestedRowRoundTrip — top-level ARRAY<ROW<...>> mapped to a POJO array
• NestedRowConverterTest#testMapWithRowValuesRoundTrip — MAP<STRING, ROW<...>> with type erasure fallback (values deserialized as InternalRow)
• NestedRowConverterTest#testRowFieldWithIncompatibleType — validates that an array type field is rejected when mapped to ROW
• NestedRowConverterTest#testRowFieldWithMapType — validates that a Map type field is rejected when mapped to ROW

API and Format

No. This change only adds internal converter logic for the existing ROW data type. No public API or storage format changes.

Documentation

Updated website/docs/apis/java-client.md:

• Added ROW | Nested POJO to the type mapping table
• Added a new "Nested POJOs (ROW Type)" section with code examples showing direct ROW fields, ARRAY<ROW>, and MAP<String, ROW> usage
• Added a note about Java type erasure limitation for MAP values

[XuQianJin-Stars]

@polyzos Hi, i already updated the pr. Please help review when you got some time.