Making scan limit of RecordBatchReaderExec handle row limit efficiently

[Kontinuation]

limit in RecordBatchReaderExec is supposed to be a number of rows. However, it is currently being applied as a number of batches. This patch fixes this by implementing a structure that keeps track of the number of iterated rows and stops early.

This patch also fixes another problem: the usage of RwLock in RecordBatchReaderExec is inappropriate, since we only access the underlying reader protected by the lock in write mode. We use a Mutex here instead, and removed unsafe impl Sync for RecordBatchReaderExec.

[Copilot]

Pull Request Overview

This PR fixes two issues in RecordBatchReaderExec: the limit parameter now correctly applies to row count instead of batch count, and replaces inappropriate RwLock usage with Mutex for better thread safety. The changes improve both correctness and performance of the record batch reader functionality.

• Implements proper row-based limiting with RowLimitedIterator that tracks consumed rows and handles batch truncation
• Replaces RwLock with Mutex and removes unsafe Sync implementation for better thread safety
• Adds comprehensive test coverage for various row limit scenarios including edge cases with empty batches

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