fix(core): restore the page cache state once per fetch_schema call

[LukasGold]

Fixes #176.

Changes

• OSW.fetch_schema reads the cache state once, enables the cache, and
  restores the state in a finally block around the loop over schema titles.
• OSW._fetch_schema no longer reads or restores the cache state. A note in
  its docstring says where that now happens.
• The importlib.reload(model) call under if fetchSchemaParam.final: stays
  unchanged.
• New unit tests in tests/test_fetch_schema_cache.py.

Rationale

• site_cache_state was a local of _fetch_schema, which runs once per title
  and again recursively per $ref. From the second title on, the snapshot
  already read the state the previous call had set, so if not site_cache_state was false and disable_cache() was never called.
• The restore was also unreachable on two paths: the early return for a
  missing schema page, and any exception. A finally in the caller covers
  both.
• _fetch_schema is private and only called from fetch_schema and from
  itself, so moving the cache handling to the public method keeps the caching
  benefit for the whole operation.
• enable_cache/disable_cache are plain boolean setters with no nesting
  counter, so a single enable and a single restore around the operation is the
  correct shape.

Verification

• 6 new tests pass.
• 5 of the 6 fail against the previous src/osw/core.py, including the test
  for a worker that enables the cache and does not restore it.
• The tests replace _fetch_schema with a stub, so they assert the cache
  contract of fetch_schema. They do not execute the body of _fetch_schema,
  which needs a wiki.

Related

• OSW.load_entity has the same save and restore pattern at
  https://github.com/OpenSemanticLab/osw-python/blob/main/src/osw/core.py#L1235
  and #L1325. It restores unconditionally at the end but not in a finally,
  so an exception still skips it. Not changed here.

[github-actions]

Release preview

Merging this PR would release v2.3.4 (current: v2.3.3).

Changelog preview (truncated)

## v2.3.4 (2026-09-18)

### Bug Fixes

- **core**: Restore the page cache state once per fetch_schema call
  ([`f09dcf6`](https://github.com/OpenSemanticLab/osw-python/commit/f09dcf6b9d0e0bf3baf04107d9e1acf56e755c8d))

### Chores

- Relicense to Apache-2.0 ([#173](https://github.com/OpenSemanticLab/osw-python/pull/173),
  [`848615c`](https://github.com/OpenSemanticLab/osw-python/commit/848615cbd1e06401922eda88231a51028440a242))

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