fix: serialize downloaded ad timestamps as schema-compliant strings (#863)

## ℹ️ Description
- Link to the related issue(s): Issue #
- Fixes drift where `pdm run app download` wrote timestamp values in
YAML-native datetime form that could violate `schemas/ad.schema.json`
string expectations.
- Ensures downloaded ads persist `created_on`/`updated_on` as
JSON-serialized ISO-8601 strings and adds a regression test validating
written YAML against the schema.

## 📋 Changes Summary
- Updated downloader save path to use `ad_cfg.model_dump(mode =
\"json\")` before writing YAML in `src/kleinanzeigen_bot/extract.py`.
- Updated existing `download_ad` unit assertion to match JSON-mode
serialization.
- Added `test_download_ad_writes_schema_compliant_yaml` in
`tests/unit/test_extract.py` that writes a real tmp YAML file and
validates it against `schemas/ad.schema.json` with `jsonschema`.
- Added dev dependency `jsonschema>=4.26.0` (and lockfile updates).
- Dependencies/config updates introduced: new dev dependency
(`jsonschema`) for full schema validation in tests.

### ⚙️ Type of Change
- [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
- [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`).
- [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

# Release Notes

* **Bug Fixes**
* Improved ad data serialization to ensure consistent JSON format when
saving ad configurations.

* **Tests**
  * Added schema validation tests to verify ad YAML output compliance.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/unit/test_extract.py

@@ -2,19 +2,28 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # SPDX-ArtifactOfProjectHomePage: https://github.com/Second-Hand-Friends/kleinanzeigen-bot/
 import json  # isort: skip
+import asyncio
 from gettext import gettext as _
 from pathlib import Path
-from typing import Any, TypedDict
+from typing import Any, Final, TypedDict
 from unittest.mock import AsyncMock, MagicMock, call, patch
 from urllib.error import URLError
 
 import pytest
+from jsonschema import Draft202012Validator
+from ruamel.yaml import YAML
 
 import kleinanzeigen_bot.extract as extract_module
 from kleinanzeigen_bot.model.ad_model import AdPartial, ContactPartial
 from kleinanzeigen_bot.model.config_model import Config, DownloadConfig
 from kleinanzeigen_bot.utils.web_scraping_mixin import Browser, By, Element
 
+SCHEMA_PATH:Final[Path] = Path(__file__).resolve().parents[2] / "schemas" / "ad.schema.json"
+
+
+def _read_text_file(path:Path) -> str:
+    return path.read_text(encoding = "utf-8")
+
 
 class _DimensionsDict(TypedDict):
     ad_attributes:str
@@ -1255,7 +1264,38 @@ class TestAdExtractorDownload:
             actual_call = mock_save_dict.call_args
             actual_path = Path(actual_call[0][0])
             assert actual_path == yaml_path
-            assert actual_call[0][1] == mock_extract_with_dir.return_value[0].model_dump()
+            assert actual_call[0][1] == mock_extract_with_dir.return_value[0].model_dump(mode = "json")
+
+    @pytest.mark.asyncio
+    async def test_download_ad_writes_schema_compliant_yaml(self, extractor:extract_module.AdExtractor, tmp_path:Path) -> None:
+        """Test that downloaded ad YAML validates against ad.schema.json."""
+        download_base = tmp_path / "downloaded-ads"
+        final_dir = download_base / "ad_12345_Test Advertisement Title"
+        yaml_path = final_dir / "ad_12345.yaml"
+        extractor.download_dir = download_base
+
+        with patch.object(extractor, "_extract_ad_page_info_with_directory_handling", new_callable = AsyncMock) as mock_extract_with_dir:
+            mock_extract_with_dir.return_value = (
+                AdPartial.model_validate(
+                    {
+                        "title": "Test Advertisement Title",
+                        "description": "Test Description",
+                        "category": "Dienstleistungen",
+                        "created_on": "2026-03-08T00:00:00+01:00",
+                        "updated_on": "2026-03-09T01:02:03+01:00",
+                    }
+                ),
+                final_dir,
+            )
+
+            await extractor.download_ad(12345)
+
+        loaded_ad = YAML(typ = "safe").load(await asyncio.to_thread(_read_text_file, yaml_path))
+        schema = json.loads(await asyncio.to_thread(_read_text_file, SCHEMA_PATH))
+
+        Draft202012Validator(schema).validate(loaded_ad)
+        assert isinstance(loaded_ad["created_on"], str)
+        assert isinstance(loaded_ad["updated_on"], str)
 
     @pytest.mark.asyncio
     # pylint: disable=protected-access