fix(core): detect CharacteristicType from the opensemantic packages too

[simontaurus]

store_entity writes the generated schema flat instead of into $defs.generated when the entity comes from the released opensemantic packages.

The check only knows the locally generated class:

if hasattr(model, "CharacteristicType") and isinstance(entity_, model.CharacteristicType):
    mode = AggregateGeneratedSchemasParamMode.DEFINITIONS_SECTION

Characteristics built from opensemantic.core.v1 derive from that package's CharacteristicType, not from the one in osw.model.entity, so the isinstance check fails and the mode stays ROOT_LEVEL. The autogenerated half then lands at the root of the schema and overwrites the manually maintained part on the next save.

Changes

• import CharacteristicType from opensemantic.core.v1 as an optional dependency, None when the package is absent
• accept either class in the isinstance check
• guard the change_id tagging in store_entity against entity models without a meta field, which the same models hit

Found while building world.opensemantic.characteristics.quantitative v0.7.0, where 952 schema files came out flat. With the patch the diff drops to the 25 files that actually changed.

[github-actions]

Release preview

Merging this PR would release v2.8.2 (current: v2.8.1).

Changelog preview (truncated)

## v2.8.2 (2026-10-10)

### Bug Fixes

- Detect CharacteristicType from the opensemantic packages too
  ([`f195a97`](https://github.com/OpenSemanticLab/osw-python/commit/f195a973b141cf830c65656ab946ca454e7309aa))

_{Preview via python-semantic-release and conventional commits.}