fix: JSON API Pagination for >25 Ads (#797)

## ℹ️ Description
*Provide a concise summary of the changes introduced in this pull
request.*

- Link to the related issue(s): Closes #789 (completes the fix started
in #793)
- **Motivation**: Fix JSON API pagination for accounts with >25 ads.
Aligns pagination logic with weidi’s approach (starts at page 1), while
hardening error handling and tests. Based on
https://github.com/weidi/kleinanzeigen-bot/pull/1.

## 📋 Changes Summary

- Added pagination helper to fetch all published ads and use it in
delete/extend/publish/update flows
- Added robust handling for malformed JSON payloads and unexpected ads
types (with translated warnings)
- Improved sell_directly extraction with pagination, bounds checks, and
shared coercion helper
- Added/updated tests for pagination and edge cases; updated assertions
to pytest.fail style

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [x] 🐞 Bug fix (non-breaking change which fixes an issue)
- [ ] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)


## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test:cov:unified`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Reliable multi-page fetching for published ads and buy-now eligibility
checks.

* **Bug Fixes**
* Safer pagination with per-page JSON handling, limits and improved
termination diagnostics; ensures pageNum is used when needed.

* **Tests**
* New comprehensive pagination tests and updates to existing tests to
reflect multi-page behavior.

* **Chores**
* Added a utility to safely coerce page numbers; minor utility signature
cleanup.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/unit/test_init.py

@@ -676,7 +676,8 @@ class TestKleinanzeigenBotBasics:
         ):
             await test_bot.publish_ads(ad_cfgs)
 
-            web_request_mock.assert_awaited_once_with(f"{test_bot.root_url}/m-meine-anzeigen-verwalten.json?sort=DEFAULT")
+            # With pagination, the URL now includes pageNum parameter
+            web_request_mock.assert_awaited_once_with(f"{test_bot.root_url}/m-meine-anzeigen-verwalten.json?sort=DEFAULT&pageNum=1")
             publish_ad_mock.assert_awaited_once_with("ad.yaml", ad_cfgs[0][1], {}, [], AdUpdateStrategy.REPLACE)
             web_await_mock.assert_awaited_once()
             delete_ad_mock.assert_awaited_once_with(ad_cfgs[0][1], [], delete_old_ads_by_title = False)