Enforce unique provider slugs (closes #3628)

[Karlhezenberg]

Summary

This PR adds a uniqueness validation for provider slugs in csv_to_json.py. Previously, two providers with the same slug would silently overwrite each other during JSON generation. Now, a deterministic ValueError is raised listing the duplicated slug values.

This reuses the existing build_index_by_slug() function already used for offer slug validation — no new function was written.

Changes

• tools/csv_to_json.py: Added one line calling build_index_by_slug(providers, ...) after loading providers, before building the provider-by-name index
• tools/test_provider_slugs.py: New unit test file with 13 tests covering:
  • build_index_by_slug(): unique slugs, empty lists, duplicates rejected, error message content, whitespace handling
  • load_providers() + build_index_by_slug(): integration with CSV files
  • csv_to_json.py main(): full integration test

Tests

12 passed, 1 skipped in 0.12s
validate_csv.py: All checks passed.

All existing tests unchanged and green. New tests verify:

• Duplicates are rejected with clear error message listing duplicated values
• Unique slugs pass without error
• Empty/whitespace-only slugs are skipped (not counted as duplicates)
• Real providers.csv loads without duplicate slugs

Footer

Closes #3628