Add GCS download mode for Tennessee diabetes import

[rohitkumarbhagat]

Summary

• Add --download_source=tn|gcs to the Tennessee diabetes downloader.
• Keep TN.gov as the default for direct execution.
• Enable GCS mode in the import manifest. - last download from TN.gov passed. Keeping it default.
• Generate yearly filenames from 2019 through the current year for both modes.
• Skip missing GCS years, fail when no files are downloaded, and propagate unexpected GCS errors.
• Log download source, destination, completion, and file size.
• Document the semi-manual GCS staging workflow.

Motivation

TN.gov blocks requests originating from Google Cloud environments. GCS staging provides a semi-manual fallback while preserving the existing processing flow.

Validation

• Added four focused unit tests for URL generation and GCS behavior.
• All focused tests pass.
• Repository lint passes.
• Completed a live GCS download and Excel-processing validation for the 2019–2022 files.

The runtime service account requires storage.objects.get access to the unresolved_mcf bucket.

[gemini-code-assist]

Code Review

This pull request introduces a semi-manual GCS fallback mode to the Tennessee diabetes data importer, allowing files to be downloaded from Google Cloud Storage when TN.gov is blocked. It updates the README, adds a --download_source flag and GCS download logic to download.py, and introduces a new test suite in download_test.py. The review feedback correctly points out a potential compatibility issue with e.add_note(), which requires Python 3.11+ and could cause runtime errors in older environments.