refactor(基础模块): 优化对clob的支持

[zhou-hao]

Note

Make CLOB encoding/decoding and null handling column-aware, replacing ClobValue with LongCharSequence and updating executors/builders (incl. Oracle) and tests accordingly.

• Core:
  • Add column-aware ValueCodec.encodeNull(ColumnMetadata) and use it in AbstractColumnMetadata and rdb/metadata/RDBColumnMetadata.
• RDB Codecs:
  • ClobValueCodec: add isClobType, return NullValue for column-aware nulls, wrap CLOB text as LongCharSequence.
  • JsonValueCodec: use column-aware nulls and LongCharSequence for CLOB columns; expand decode support (e.g., Reader, typed blobs) and handle null targets.
  • New LongCharSequence replaces prior CLOB wrapper; used for streaming/Clob binding.
  • NullValue: store explicit type, improve getType() and toString(), add of(DataType) helper.
• Executors:
  • JDBC: bind LongCharSequence via setCharacterStream; honor NullValue SQL types.
  • R2DBC: bind LongCharSequence as Clob; bind NullValue by Java type, special-case CLOB nulls.
• Oracle SQL Builders:
  • OracleBatchUpsertOperator and OracleInsertSqlBuilder: encode values per column; for batched CLOB nulls, substitute empty LongCharSequence to avoid null binding.
• Tests/Infra:
  • Add batch save tests (sync/reactive) and adjust Oracle connection providers; minor test formatting tweaks.

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[cursor]

Bug: Null Pointers: Inconsistent Null Handling.

The getType() method can throw NullPointerException when type is null and dataType is also null, since it calls dataType.getJavaType() without null-checking dataType. The @NonNull annotation was removed from the dataType field, allowing null values, but the getter doesn't handle this case.