chore: various compatibility hints changes

This is a combination of various work done on the compatibility hints.

The compatibility hints are still a bit "work in progress", so it's
considered OK to do breaking changes here even on minor-versions.
It's used mostly in the test code, but also internally in the library
to work around various compatibility problems.

The aim of the work is to get rid of all the old "compatibility issue
flags", all the new "features" should be tested by the
caldav-server-tester, this file should contain everything needed by
the code and by the tests, in addition to other useful/interessting
features checked by the caldav-server-tester project.

This commit is a combination of many commits, many of them
AI-generated.  Maintaining this file is tedious work and not part of
the core logic.  Quite some of the prompts have gone missing, quite
much of the work has been done in the caldav-server-tester project.

I do believe the changes have been through sufficient scrutiny.  Claude
does have a history of messing up this file - hallucinating up RFC
references, and frequently flipping compatibility support instead of
doing research on why the feature support is "fragile".

Here are some of the prompts:

prompt: (in caldav-server-tester project ) Create a check for freebusy-query.rfc6638
followup-prompt: this being a RFC6638-thing, it's meant to be used in a multi-user-scenario, so consider usage of ServerQuirkChecker.extra_principals.  (Also, rename the old FreeBusy check class with a name consistent with the new FreeBusy class)
followup-prompt: rename the feature to scheduling.freebusy-query (not freebusy-query.rfc6638)
followup-prompt: but then freebusy-query.rfc4791 becomes a bit redundant, it can be removed and merged into freebusy-query, with the description of freebusy-query emphasizing that it's a check for freebusy as given in rfc4791 (with reference to scheduling.freebusy-query for the rfc6638-variant)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen

caldav/compatibility_hints.py

@@ -1106,6 +1106,8 @@ def dotted_feature_set_list(self, compact=False):
     "search.recurrences.expanded.exception": False,
     'old_flags': ['vtodo_datesearch_nodtstart_task_is_skipped'],
     'test-calendar': {'cleanup-regime': 'wipe-calendar'},
+    'scheduling.schedule-tag': False,
+    'scheduling.mailbox.inbox-delivery': False,
 }
 
 baikal =  { ## version 0.10.1