[arrow] Fix incorrect index usage in ArrowFieldWriters.TimeWriter

[guluo2016]

Purpose

Linked issue: close #6767

Tests

API and Format

Documentation

[guluo2016]

@yuzelin @Zouxxyy @JingsongLi Can you review this pr when you have time, thanks!
If I missed anything, please let me know, thanks.

[kaori-seasons]

Current situation: Similar index mixing (using i to read source data, or using row to check the Arrow vector) can easily occur repeatedly in multiple field writers, leading to hidden bugs.

Recommendation:

Recommendation: Define and strictly distinguish between sourceIndex (e.g., rowIndex = startIndex + i) and targetIndex(i) in the implementation.

Within the loop body of each writer, use sourceIndex to read data from the ColumnVector, and use targetIndex to write data to the ArrowVector; use sourceIndex to check for null values.

Example:

// Assume startIndex, batchRows, columnVector, and timeMilliVector are known.
for (int i = 0; i < batchRows; i++) {
    int sourceIndex = startIndex + i; // 从 columnVector 读取的索引
    int targetIndex = i;              // 写到 Arrow 向量的位置
    if (columnVector.isNullAt(sourceIndex)) {
        timeMilliVector.setNull(targetIndex);
    } else {
        int value = ((IntColumnVector) columnVector).getInt(sourceIndex);
        timeMilliVector.setSafe(targetIndex, value);
    }
}

[guluo2016]

I didn’t modify ArrowFieldWriters.java to keep it consistent with the rest of the code.
Updating the test case to improve readability, thanks for the review