Add handler tests for GET /api/v1/data-version/{lang}

[DeleMike]

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Description

Tests for the endpoint clients should use to decide whether to re-download a language pack.

Covers the <datatype>_last_modified map shape, the 400/404/500 paths, an empty map for a language with zero data types, and the 1970-01-01 fallback when a table has no lastModified column.

A wrong or stale date here makes clients either miss updates or re-download everything, and neither failure is visible server-side.

Contribution

No response