fix(news): revert get_item() to use get_items() + filter

Reverts the "perf(news): use direct API endpoint for get_item()" change
from commit 92c4bf3 which incorrectly assumed GET /items/{itemId} exists.

The News API (v1-2, v1-3, v2) does not provide a direct endpoint to
retrieve individual items. The only /items/{itemId} routes are POST
operations for marking items read/unread/starred.

Changes:
- Restore original get_item() implementation that fetches all items
  and filters in Python
- Update exception from HTTPStatusError to ValueError
- Restore documentation explaining API limitation
- Update unit tests to mock get_items() instead of _make_request()
- Add test for ValueError when item not found

Fixes vector processor 405 errors when indexing news items.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/client/news.py

@@ -228,6 +228,10 @@ class NewsClient(BaseNextcloudClient):
     async def get_item(self, item_id: int) -> dict[str, Any]:
         """Get a specific item by ID.
 
+        Note: The News API doesn't have a direct single-item endpoint,
+        so we fetch all items and filter. For efficiency, consider
+        caching or using get_items with specific feed if known.
+
         Args:
             item_id: Item ID
 
@@ -235,10 +239,15 @@ class NewsClient(BaseNextcloudClient):
             Item data
 
         Raises:
-            HTTPStatusError: 404 if item not found
+            ValueError: If item not found
         """
-        response = await self._make_request("GET", f"{self.API_BASE}/items/{item_id}")
-        return response.json()
+        # Fetch all items and find the one we need
+        # This is inefficient but the API doesn't provide a direct endpoint
+        items = await self.get_items(batch_size=-1, get_read=True)
+        for item in items:
+            if item.get("id") == item_id:
+                return item
+        raise ValueError(f"Item {item_id} not found")
 
     async def get_updated_items(
         self,