feat(versioning): ETag helper module + integration tests (T055)

Helper module that derives the strong-validator ``ETag`` value from
an entity's current live ``version_uuid`` and attaches it to a
Flask response. Two functions:

- ``set_version_etag(response, version_uuid)`` — direct path used by
  PUT handlers that already compute ``new_version_uuid`` (see the
  REST API commit two prior). Cheap; no extra query.
- ``set_version_etag_by_uuid(response, model_cls, entity_uuid)`` —
  used by version endpoints that operate on ``entity_uuid``; looks
  up ``entity_id`` then derives ``version_uuid`` via ``VersionDAO``.
  Costs one extra ``SELECT id WHERE uuid = ?``; documented in the
  docstring so callers prefer the cheap variant when they have the
  id already.

Integration tests cover all three entity types and four endpoint
shapes (entity GET, save PUT, version-list GET, single-version GET)
plus the entity-with-no-versions edge case (header is correctly
absent).

The ETag is wired into the API endpoints in the REST-API commit
(group 3) and the CORS ``expose_headers: ["ETag"]`` ships with the
retention commit (group 4) since both touch ``superset/config.py``.
Locking enforcement (``If-Match`` → 412) is explicitly NOT in this
change — deferred to the follow-up UI SIP per Open Question §7.
``ETag`` is informational in v1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>