Hard-delete all test cases with the given tag.

Permanently removes all test cases created with the specified tag (and all their child entities) from the database.

Scoped cleanup: Each CI pipeline run should create cases with a unique tag (e.g. suite-run-{uuid}) and use this endpoint at the end of the run to clean up, without affecting cases from parallel pipeline runs.

This is a hard-delete. Rows are removed entirely with no recovery path. This is intentional for CI environments.

Guards:

• Returns 400 if any matched case is classified as Production. The entire operation is rejected and nothing is deleted.
• Returns 400 if the tag query parameter is missing or blank.

Idempotent: Returns 204 even if no cases match the tag — no error is raised.