Support negated searches

[tobixen]

The CalDAV-protocol supports negated text matches, but this is not properly implemented in the current search functionality.

This requires some work:

• In the caldav-server-tester project, it's needed to do some testing to check if the server supports it or not
• In the icalendar-searcher project, it's needed to implement support for the != operator
• In search.py, the build_search_xml_query should be fixed to support negated text matches
• In search.py, we need a workaround for removing the attribute from the search and then do client-side filtering in cases where the server does not support negated text matches.
• It's needed to add both unit tests and functional tests

[tobixen]

Claude made an estimate that this task will take 12-16 hours. Let me think a bit:

• The icaledar-searcher library does not support the != operator yet. Start by adding test code it (trivial, one hour)
• The icalendar-searcher library has to be fixed to support != (trivial, one hour)
• Unit tests on the modifications needed in build_search_xml_query. It may be non-trivial making a good test, but it should be easy making a simplified test - and if it's too complicated it may be skipped (one hour)
• build_search_xml_query in search.py has to be modified to support. This should also be relatively trivial (search for if self._property_operator[property] == "undef" and add the logic in that area). One hour.
• Does the server supports the logic? A new test case has to be made in the calendar-server-checker. (relatively trivial, should be one hour).
• Test all the servers we have available for this feature, adjust the compatibility_hints.py. Time-consuming, but trivial. Two hours.
• Fix functional tests in caldav_client.py. One hour.
• Fix the CalDAVSearcher to fall back to client side search for servers not supporting negated searches (search.py is already ripe for refactoring #580 needs to be done first) . One hour.
• Fight with all the servers and bang on things until the test code passes. Slightly non-trivial and time consuming. Two hours.

That adds up to 10 hours, including the work done on the sub-projects (caldav-server-tester, icalendar-searcher). Possibly less.