Short URL not working after moving page to another subwiki

[oanalavinia]

Steps to reproduce :

1. Create a short url for a page from the main wiki
   http://localhost:8080/xwiki/short/{pageID}
2. Move this page to another subwiki
3. Try to create a short url for this moved page
   http://localhost:8080/xwiki/short/{wikiName}/{pageID}
4. Observe the 2 links and try to access them both

Expected result:
Both URL will redirect to the same page, which is now in a subwiki

Actual result:
The initial URL will not redirect anymore to the page. This will result in broken links

Note that it is normal that the links are different, since the URL needs to contain the wiki id (used after for finding the page). The actual problem is that the initial link will not work anymore

This might be the expected behavior (to not support pages moved to other wikis), but we need to check if there isn't a way to solve it or simply mention it in the documentation

[oanalavinia]

Investigation:
Note that initially we had the same link structure for all wikis, before implementing #14 , but there were issues with the SOLR query (used for cross-wiki queries) which resulted in empty results sometimes. This is why we include the wiki name in the URL now.
So to solve this issue, I am thinking that we could add an extra step in here https://github.com/xwikisas/application-urlshortener/blob/main/application-urlshortener-default/src/main/java/com/xwiki/urlshortener/internal/URLShortenerResourceReferenceHandler.java#L91 . Basically, after trying to find the page based on the given wiki, try to see if SOLR will find it somewhere else. So when accessing http://localhost:8080/xwiki/short/{pageID}, even if it the classic query won't find it in the main wiki, it will find the page in the current subwiki.
This needs to be tested of course, there might be issues (maybe we can also understand better why SOLR had problems)

[KebabRonin]

Subwikis are stored in different databases, and SOLR is the only language which can query all at once. If searching across all wikis with solr, I think shadowing could become a problem (same pageID in 3 subwikis, which one to choose?).

As a possible solution, we could add the subwiki as part of the pageID (and query all subwikis with solr)

Automatic Redirect (Not Applicable)

The [automatic redirect mechanism](https://github.com/xwiki/xwiki-platform/blob/71f45383656121eee8e7b2762c427f2dc0379ccc/xwiki-platform-core/xwiki-platform-refactoring/xwiki-platform-refactoring-default/src/main/java/org/xwiki/refactoring/internal/DefaultModelBridge.java#L242) could also be reused for this purpose, since it gives the user the power to decide whether to break the URLShortener or not.

Essentially, the old page still exists but it is hidden, and contains a RedirectClass. By leaving the URLShortenerClass behind on the old page, the short URL still works and the redirect is handled by the RedirectClass, which functions across wikis.

It could be implemented by using a Listener for moving the object from the new page back to the old page, whenever a RedirectClass object is added (since the new page is specified in the RedirectClass).

This may also fix cases when the moved page conflicts with an existing short URL, like in the following scenario:

1. Page A on the main wiki has short URL '12345' (so short url short/12345)
2. Page B on a subwiki has short URL '12345' (so short url short/subwiki/12345)
3. Page B is moved to the main wiki
4. Now short/12345 could point to either A or B (since B retains its old URL across the wiki move)

With the automatic redirect solution, the URLShortener object isn't moved to the new page. So if the URL is re-generated on the main wiki, a new URL will be generated with no conflict with Page A.