fix: testCheckCompatibility now catches subfeatures collapsed by compact=True

compact=True triggers collapse() which mutates _server_features by removing
subfeatures that roll up into a parent.  The guard
  `feature not in fo._server_features`
then wrongly treats those tested-but-collapsed features as "never tested",
silently skipping the assertion.

Fix: snapshot _server_features.keys() before calling dotted_feature_set_list
(compact=True), and use that snapshot in the guard.

Also update compatibility matrices for Xandikos and Stalwart:
search.recurrences.expanded.exception is now observed as supported on both
servers (the previously documented bugs appear to be fixed in current versions).

prompt: Running the compatibility tests (which again runs code from ~/caldav-server-tester), I find this: For servers Xandikos, Stalwart: search.recurrences.expanded.exception found to be supported, compatibility matrix says it's not supported. Despite this, the compatibility test passes - why? The test should break when differences are found, except for if feature support is "fragile" or "unknown". please fix

AI Prompts:
claude-sonnet-4-6: In this branch, the following tests are broken: FAILED tests/test_async_integration.py::TestAsyncForOx::test_object_by_uid - caldav.lib.error.PutError: PutError at '409 Conflict FAILED tests/test_caldav.py::TestForServerDavical::testRecurringDateWithExceptionSearch - KeyError: 'RECURRENCE-ID' FAILED tests/test_caldav.py::TestForServerCyrus::testRecurringDateWithExceptionSearch - KeyError: 'RECURRENCE-ID' ERROR tests/test_caldav.py::TestForServerBedework::testSetCalendarProperties - caldav.lib.error.NotFoundError: NotFoundError at '404 Not Found ERROR tests/test_caldav.py::TestForServerCCS::testSetCalendarProperties - caldav.lib.error.NotFoundError: NotFoundError at '404 Not Found ERROR tests/test_caldav.py::TestForServerSOGo::testSetCalendarProperties - caldav.lib.error.NotFoundError: NotFoundError at '404 Not Found  in the master branch those two tests are broken: FAILED tests/test_caldav.py::TestForServerDavical::testRecurringDateWithExceptionSearch - KeyError: 'RECURRENCE-ID' FAILED tests/test_caldav.py::TestForServerCyrus::testRecurringDateWithExceptionSearch - KeyError: 'RECURRENCE-ID' ... and that's weird, because I'm pretty sure they passed before I pushed the changes and passed at github before I approved the pull request. please do some research
claude-sonnet-4-6: Oh, "this branch" was meant to be async-github-testruns.  I've checked it out now.
claude-sonnet-4-6: Running the compatibility tests (which again runs code from ~/caldav-server-tester), I find this:  For servers Xandikos, Stalwart: search.recurrences.expanded.exception found to be supported, compatibility matrix says it's not supported.  Despite this, the compatibility test passes - why?  The test should break when differences are found, except for if feature support is "fragile" or "unknown".
claude-sonnet-4-6: please fix

Author: tobixen

caldav/compatibility_hints.py

@@ -907,9 +907,8 @@ def dotted_feature_set_list(self, compact=False):
     ## Principal property search returns 403 (not implemented)
     "principal-search": "ungraceful",
 
-    ## Server-side recurrence expansion for event exceptions is still broken;
     ## VTODO RRULE expansion was fixed in xandikos PR #627 (released in 0.3.7).
-    "search.recurrences.expanded.exception": "unsupported",
+    ## Exception expansion (CALDAV:expand with EXDATE/RECURRENCE-ID) is now also supported.
 
     ## Open-start time-range searches (no lower bound) crash xandikos 0.3.7 with a
     ## 500 Internal Server Error (OverflowError: date value out of range in icalendar.py
@@ -1423,10 +1422,7 @@ def dotted_feature_set_list(self, compact=False):
     ## Stalwart returns the recurring todo in search results but doesn't return the
     ## RRULE intact, so client-side expansion can't expand it to specific occurrences.
     'search.recurrences.includes-implicit.todo': {'support': 'fragile'},
-    ## Stalwart doesn't handle exceptions properly in server-side CALDAV:expand:
-    ## returns 3 items instead of 2 for a recurring event with one exception
-    ## (the exception is stored as a separate object and returned twice).
-    'search.recurrences.expanded.exception': False,
+    ## Stalwart correctly handles exceptions in server-side CALDAV:expand (observed supported).
     ## Stalwart stores master+exception VEVENTs as a single resource with 2 VEVENTs.
     'save-load.event.recurrences.exception': {'support': 'full'},
     'search.time-range.open': True,

tests/test_caldav.py

@@ -1535,6 +1535,10 @@ def testCheckCompatibility(self, request) -> None:
         fe = self.caldav.features
 
         ## dotted list expected and observed
+        ## Snapshot checked features before compact=True calls collapse(), which
+        ## mutates _server_features by removing subfeatures that collapse into
+        ## their parent — making tested features look like untested ones.
+        checked_features = set(fo._server_features.keys())
         observed = fo.dotted_feature_set_list(compact=True)
         expected = fe.dotted_feature_set_list(compact=True)
 
@@ -1547,7 +1551,7 @@ def testCheckCompatibility(self, request) -> None:
                 continue
             ## Skip features the checker never explicitly tested -
             ## the observation would just be a default, not a real result
-            if feature not in observed and feature not in fo._server_features:
+            if feature not in observed and feature not in checked_features:
                 continue
             type_ = fo.find_feature(feature).get("type", "server-feature")
             if type_ in (