[IndiaNSS_HealthAilments] Made changes in the download script and pvmap , added goldens checks

[Krishnam24maheshwari]

This PR resolves multiple pipeline and data quality issues for the IndiaNSS_HealthAilments import :

1. Restores missing "60+" senior demographic mapping in pvmap.csv that previously caused 992 lint errors, 51 dropped StatVars, and demographic attribution corruption.
2. Cleans up download_script.py and fixes error handling by removing dead duplicate code, enforcing deterministic script-relative output paths, and handling retry exhaustion gracefully with explicit RuntimeError.
3. Synchronizes and expands download_script_test.py to achieve full mock unit test coverage.
4. Added goldens rules (golden_observations.csv) goldens_summary_report is missing because non of the statvars generated in this import is there in "gs://unresolved_mcf/import_validation/nl_statvars.csv".
5. Aligns manifest.json and README.md with standard naming conventions and output counters.

Differ summary to verify data - https://pantheon.corp.google.com/storage/browser/_details/datcom-import-test/statvar_imports/india_ndap/india_nss_health_ailments/IndiaNSS_HealthAilments/2026_08_20T06_39_52_485565_07_00/input0/validation/differ_summary.json;tab=live_object?pageState=(%22StorageObjectListTable%22:(%22f%22:%22%255B%255D%22))&project=datcom-infosys-dev

Validation_output - https://pantheon.corp.google.com/storage/browser/_details/datcom-import-test/statvar_imports/india_ndap/india_nss_health_ailments/IndiaNSS_HealthAilments/2026_08_20T06_39_52_485565_07_00/input0/validation/validation_output.csv;tab=live_object?pageState=(%22StorageObjectListTable%22:(%22f%22:%22%255B%255D%22))&project=datcom-infosys-dev

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request cleans up redundant code in download_script.py, updates the property-value mapping for age groups in india_nss_health_ailments_pvmap.csv, and updates the output path in manifest.json to use PascalCase. Additionally, it introduces a validation configuration (validation_config.json) and a golden observations dataset (golden_observations.csv) to verify output data integrity. There are no review comments, so I have no feedback to provide.

[Krishnam24maheshwari]

Made changes accordingly