fix(core): verify that store_entity actually created the page

[LukasGold]

Fixes #175.

Changes

• New StoreEntityParam.verify_write, default True.
• store_entity collects the titles it sent an edit for and queries their
  existence after the upload. A page that is absent is removed from
  StoreEntityResult.pages and added to StoreEntityResult.failed as a new
  OSW.PageNotCreatedError, which makes store_entity raise
  StoreEntityPartialError through the existing path.
• New OSW._query_missing_page_titles(titles): one action=query&prop=info
  request per 50 titles, mapping normalized titles back to the requested form.
• New OSW._get_missing_page_titles(titles, confirm_delay_s=5): queries a
  second time, 5 seconds later, for the titles the first query reported as
  missing.
• A verification query that raises is caught and logged. The pages stay in
  StoreEntityResult.pages.
• The comment claiming store_entity_ only records created_pages on success
  now says what actually happens.

Rationale

• created_pages[page.title] = page runs for every page that reaches the last
  statement of store_entity_. That only means no exception was raised. It
  does not mean the page exists.
• page.changed is no help: it was True in the reported failing case as
  well.
• The query goes to the MediaWiki API directly and not through
  WtSite.get_page, because the page cache would answer with the state from
  before the write.
• Batching by 50 titles keeps the cost at one extra request per 50 stored
  pages rather than one per page.
• The second query prevents the opposite defect: a read answered by a database
  replica that does not have the write yet would otherwise fail a store that
  did apply. A page still absent seconds later is not explained by that lag.
• A failed verification query is not evidence that the writes failed, and
  letting it propagate would discard the results the call had already
  collected. Reporting the pages and logging the error keeps more information
  than either alternative.
• Reusing failed and StoreEntityPartialError means callers that already
  handle partial failures need no change, and result.stored stays truthful.
• Verification is skipped when offline is True and for pages the overwrite
  policy kept unedited, since no write was sent for those.

Behaviour change

• A store that silently did not apply now raises StoreEntityPartialError
  instead of returning normally. Callers that want the previous behaviour can
  pass verify_write=False.
• Every online store_entity call that edits at least one page makes one
  additional API request per 50 edited pages, and waits 5 seconds plus one
  further request if a page is reported as missing.

Verification

• 10 tests in tests/test_store_entity_verify.py cover the absent page, the
  mixed batch, both skip conditions, the confirmation query, a page that
  appears only on the second query, a failing query, the batching by 50 and the
  title normalization.
• Full unit suite: 217 passed. tests/test_store_entity_failure.py and
  tests/test_overwrite_policy.py construct an OSW without a real site, so
  they now stub _get_missing_page_titles to report every page as existing.
  Their assertions are unchanged.
• The added request count was not measured against a live instance. It is
  ceil(edited_pages / 50) per call by construction.
• The server-side reason why such a write is rejected is still not identified.
  This change makes the library report the outcome correctly, it does not fix
  the write.

[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**: Confirm a missing page before reporting a failed store
  ([`5756436`](https://github.com/OpenSemanticLab/osw-python/commit/5756436e08ba95be3e177c6a78c424bfdba756c8))

- **core**: Verify that store_entity actually created the page
  ([`f84a529`](https://github.com/OpenSemanticLab/osw-python/commit/f84a5297c605fc458783e5497edeb1f97113053b))

### 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.}