refactor(realtime): move offset handling to Paimon readers

[lxy-9602]

Purpose

Linked issue: #158

This PR moves real-time offset handling from RealtimeStore plugins into the Paimon read and write layers.

The main changes are:

• Remove offset_begin from RealtimeStore::CreateQueryReaders. Store plugins now return candidate rows without applying visibility offsets.
• Remove enable_predicate_pushdown from RealtimeQueryContext. Callers decide whether a predicate should be passed to the plugin.
• Add _REALTIME_OFFSET to append-mode transport batches before they are handed to the real-time store.
• Add RealtimeOffsetBatchReader to filter rows by the visible half-open offset range and remove _REALTIME_OFFSET from query output.
• Reuse RealtimeOffsetBatchReader and KeyValueDataFileRecordReader for primary-key real-time reads.
• Generalize KeyValueDataFileRecordReader to accept a BatchReader. File row positions remain available when the wrapped reader is a FileBatchReader.
• Resolve primary-key transport fields by name, making their field order insignificant and removing unnecessary upfront schema and offset coverage validation.
• Simplify the built-in Arrow real-time store query reader to return candidate batches with an all-valid bitmap.

Tests

Added RealtimeOffsetBatchReaderTest.

API and Format

This PR changes the public real-time store plugin API:

• RealtimeStore::CreateQueryReaders no longer accepts offset_begin.
• RealtimeQueryContext no longer contains enable_predicate_pushdown.
• Append-mode transport batches now contain a leading _REALTIME_OFFSET field.
• Primary-key transport fields are resolved by name rather than a fixed order.

Documentation

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