OECD_Waste_Water_Treatment cloud job fix and goldens

[saanikaaa]

The PR is raised to fix OECD_Waste_Water_Treatment import cloud job which was failing due to deletions and also to add the golden test for the same.

The auto-refresh job was failing due to 6.87% (512) deleted records, which triggered a validation error. Verified against the source that deletions are as per source itself. More details with attached screenshot can be found here.

To fix this we have pulled all the deleted 512 data points from prod and uploaded to GCP Bucket. A common script is created "merged_historical_data.py" which helps in merging the output csv generated from statvar_processor and append/de-duplicate historical data to it for fullness of output data. This script can be used in future for merging purpose for the imports which are generated by statvar processor.

Differ: https://screenshot.googleplex.com/3vZEs7Q9NGEsLxg - Here we can see there are no deletions. Only points modification and additions. These modification are validated - Link

SOP: Link

PR_CHECKLIST: Link

[gemini-code-assist]

Code Review

This pull request introduces a new utility script merge_historical_data.py to reconcile and merge current wastewater treatment data with historical records, alongside adding validation configurations and golden datasets. Feedback on the script highlights critical improvements: normalizing key columns to strings to prevent merge mismatches due to differing data types, filtering the merged DataFrame directly to avoid index alignment bugs, and utilizing file_util.FileIO to ensure reliable writing to Google Cloud Storage paths.