feat: check for missing initial recurrence-id

RFC 4791 §9.6.5: non-initial expanded instances MUST include RECURRENCE-ID;
the initial instance MAY omit it.  Add a check that queries a range covering
both a regular occurrence (Feb 12) and an exception (Feb 13) with server-side
expand, then reports whether the initial instance includes RECURRENCE-ID.

It was finally decided to add this as annotation in the behaviour
field of the search.recurrences.expanded.event.

The only known instance was Cyrus, but the behaviour was modified
shortly after writing this test.  I'm not sure what the author of the
RFC have been thinking about here - in my head it does not make sense
to specifically allow the first recurrence in an expanded recurrence
set to be without a RECURRENCE-ID.

AI-generated.

prompt: make a test in caldav-server-tester to verify the cyrus expansion quirk; also check if there is any other places in the caldav library that needs fixing to reflect that the first instance in a recurrence set may be missing the RECURRENCE-ID
prompt: the checking should still be done, behaviour should still mark this behaviour, only that it should state supported instead of quirk
prompt: search.recurrences.expanded.recurrence-id was added in one of the last commits, it is not defined in compatibility_hints.py in the caldav project and should not exist. Please clean up. The correct behaviour of the checker is to add a behaviour field to search.recurrences.expanded if the first recurrence is missing the recurrence-id field.

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

Author: tobixen

src/caldav_server_tester/checks.py

@@ -1365,6 +1365,30 @@ def _run_check(self):
             == datetime(2000, 2, 13, 12, tzinfo=utc),
         )
 
+        ## RFC 4791 §9.6.5: non-initial expanded instances MUST include RECURRENCE-ID;
+        ## the initial instance MAY omit it.  Query a range covering both the regular
+        ## Feb 12 occurrence and the Feb 13 exception to detect servers that omit
+        ## RECURRENCE-ID on the initial instance, and annotate the expanded.event feature.
+        multi = cal.search(
+            start=datetime(2000, 2, 12, tzinfo=utc),
+            end=datetime(2000, 2, 14, tzinfo=utc),
+            event=True,
+            server_expand=True,
+            post_filter=False,
+        )
+        if len(multi) == 2:
+            initial = next(
+                (e for e in multi if e.component["dtstart"] == datetime(2000, 2, 12, 12, 0, 0, tzinfo=utc)),
+                None,
+            )
+            if initial is not None and getattr(initial.component.get("RECURRENCE-ID"), "dt", None) is None:
+                self.set_feature(
+                    "search.recurrences.expanded.event",
+                    {
+                        "behaviour": "initial occurrence lacks RECURRENCE-ID; RFC 4791 §9.6.5 permits this — clients must fall back to DTSTART"
+                    },
+                )
+
 
 class CheckCaseSensitiveSearch(Check):
     """