[format] Fix Parquet delta length reads across vector batches

[ArnavBalyan]

Purpose

• Parquet delta encoding stores variable string length based on position on encoded page.
• For 1 page -> many vector batches, this incorrectly uses the output position had been reset and causes later strings to be truncated.
• Ensure we can correctly track the variable length string
• This is a bug fix ported from Spark

Tests

• UT

[ArnavBalyan]

cc @JingsongLi thanks!