fix: use native page xpath api for xpath selectors (#853)

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

- Link to the related issue(s): n/a
- Describe the motivation and context for this change.
This replaces the stacked XPath work from #845 with a standalone fix
from `main`. It makes `By.XPATH` use the native page XPath API instead
of routing XPath selectors through text lookup.

## 📋 Changes Summary

- Add private XPath helpers in `WebScrapingMixin` for first-match and
all-match lookups.
- Route `By.XPATH` in `_web_find_once()` and `_web_find_all_once()`
through `page.xpath(...)`.
- Add unit coverage for XPath helper behavior, empty results, and
unsupported parent scoping.
- No configuration changes or new dependencies.

### ⚙️ 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`).
- [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

* **Refactoring**
* Improved web scraping element selection reliability through
streamlined XPath operations and better internal helper methods.

* **Tests**
* Added comprehensive unit tests for XPath-based element lookup
operations to ensure consistent behavior.

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

src/kleinanzeigen_bot/utils/web_scraping_mixin.py

@@ -1009,6 +1009,17 @@ class WebScrapingMixin:
         # Return primitive values as-is
         return data
 
+    async def _xpath_first(self, selector_value:str) -> Element | None:
+        matches = await self.page.xpath(selector_value, timeout = 0)
+        for match in matches:
+            if match is not None:
+                return cast(Element, match)
+        return None
+
+    async def _xpath_all(self, selector_value:str) -> list[Element]:
+        matches = await self.page.xpath(selector_value, timeout = 0)
+        return [cast(Element, match) for match in matches if match is not None]
+
     async def web_find(self, selector_type:By, selector_value:str, *, parent:Element | None = None, timeout:int | float | None = None) -> Element:
         """
         Locates an HTML element by the given selector type and value.
@@ -1084,7 +1095,7 @@ class WebScrapingMixin:
             case By.XPATH:
                 ensure(not parent, f"Specifying a parent element currently not supported with selector type: {selector_type}")
                 return await self.web_await(
-                    lambda: self.page.find_element_by_text(selector_value, best_match = True),
+                    lambda: self._xpath_first(selector_value),
                     timeout = timeout,
                     timeout_error_message = f"No HTML element found using XPath '{selector_value}'{timeout_suffix}",
                     apply_multiplier = False,
@@ -1129,7 +1140,7 @@ class WebScrapingMixin:
             case By.XPATH:
                 ensure(not parent, f"Specifying a parent element currently not supported with selector type: {selector_type}")
                 return await self.web_await(
-                    lambda: self.page.find_elements_by_text(selector_value),
+                    lambda: self._xpath_all(selector_value),
                     timeout = timeout,
                     timeout_error_message = f"No HTML elements found using XPath '{selector_value}'{timeout_suffix}",
                     apply_multiplier = False,