[weather-sp]: Skip creating split files that already exist

## Problem
Currently in `weather-sp`, the file splitter skips processing an input file only when all its split children already exist. If even one of the split files is missing (for example, due to a partial failure or interrupted run), the splitter re-splits the entire file and overwrites all the previously created children.

This leads to redundant processing and unnecessary I/O, especially when re-running pipelines to fill in missing data.

## Proposed Solution
Add functionality to skip the creation of individual split files if they already exist in the output location.

Instead of an all-or-nothing approach at the input file level, the splitter should:

1. Identify which specific split files are expected to be produced.
2. Check if each file already exists.
3. Only generate and upload the missing files, leaving the existing ones intact.