fix: keep the stored jsonschema when store_entity regenerates it

[LukasGold]

Closes #215

Changes

• store_entity passes the stored jsonschema slot as target_schema=. The old keyword schema= is not a field of AggregateGeneratedSchemasParam, so the aggregation always started from {} and dropped the hand-written schema.
• A page without a jsonschema slot starts from {}, as before.
• New function remove_generated_schema_section() in src/osw/core.py. It removes $defs/generated and the allOf entry {"$ref": "#/$defs/generated"}, as osl.util.postProcessPage in PageBot.js does.
• store_entity calls it in DEFINITIONS_SECTION mode (Characteristics) before the aggregation, so the generated section is replaced instead of merged. It is called only when template generation produced at least one generated schema, so a failed generation keeps the stored schema.

Tests

New tests/test_store_entity_keeps_jsonschema.py, 5 tests. They run store_entity with offline=True and a fake site that serves the meta category, so no wiki is needed.

• ROOT_LEVEL keeps hand-written properties.
• DEFINITIONS_SECTION keeps $defs entries outside the generated section.
• A second save with a different template drops a stale required and type, and $comment names the meta category once.
• A template that produces invalid JSON leaves the stored schema unchanged.
• A repeated ROOT_LEVEL save gives the same schema.

The first four tests failed on main before the fix. With only the target_schema= change, the stale-section test still failed at required.

Local gate: make check passed, make test reported 805 passed.

Not in this PR

As noted in the issue, resetGeneratedJsonLdContext and stripRootMembersSupersededByGenerated from PageBot.js are not ported. In ROOT_LEVEL mode a root entry that a later template no longer produces stays in the schema.

[github-actions]

Release preview

Merging this PR would release v2.8.3 (current: v2.8.2).

Changelog preview (truncated)

## v2.8.3 (2026-10-16)

### Bug Fixes

- Keep the stored jsonschema when store_entity regenerates it
  ([`9b76a8f`](https://github.com/OpenSemanticLab/osw-python/commit/9b76a8fbeb813b34c4881b54d329e9e837b08f95))

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