feat: add save-load.mutable feature replacing the old no_overwrite flag

Add 'save-load.mutable' to the FEATURES dict in compatibility_hints.py:
it indicates that a calendar object resource can be modified and PUT
back to the server (the server must reflect the change on the next
GET/REPORT).  Defaults to 'full' as virtually all servers support this;
the check exists to surface rare exceptions like Google Calendar's
legacy CalDAV API.

Replace every use of the old 'no_overwrite' quirk flag in test_caldav.py
with the new feature:
  skip_on_compatibility_flag("no_overwrite") → skip_unless_support("save-load.mutable")
  if not check_compatibility_flag("no_overwrite"): → if is_supported("save-load.mutable"):

prompt: Create tests for a new feature checking if it's possible to
overwrite a calendar event with new data. Should replace the old
"overwrite" compatibility flag. (Perhaps this "incompatibility" was due
to the caldav library not supporting etags and not updating from no
sequence to SEQUENCE:1 ... i somehow doubt we'll find any servers not
supporting this feature, but we should have the check anyway).
load-save.mutable ? Update the compatibility_hints.py in
/tmp/caldav-async-tests/ and update the tests there as well.

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

Author: tobixen

caldav/compatibility_hints.py

@@ -177,6 +177,10 @@ class FeatureSet:
             "default": {"support": "full"},
             "links": ["https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.4.5"],
         },
+        "save-load.mutable": {
+            "description": "A saved calendar object resource can be modified and PUT back to the server; the server accepts the update and returns the modified data on the next GET/REPORT. When 'unsupported', the server treats calendar objects as immutable after initial creation (e.g. Google Calendar's legacy CalDAV API). Replaces the old 'no_overwrite' compatibility flag.",
+            "default": {"support": "full"},
+        },
         "search": {
             "description": "calendar MUST support searching for objects using the REPORT method, as specified in RFC4791, section 7",
             "links": ["https://datatracker.ietf.org/doc/html/rfc4791#section-7"],

tests/test_caldav.py

@@ -2103,7 +2103,7 @@ def testObjectBySyncToken(self):
             assert len(list(my_changed_objects)) == 0
 
         ## I was unable to run the rest of the tests towards Google using their legacy caldav API
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         ## MODIFYING an object
         if is_time_based:
@@ -2234,7 +2234,7 @@ def testSync(self):
             time.sleep(1)
 
         ## I was unable to run the rest of the tests towards Google using their legacy caldav API
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         ## MODIFYING an object
         obj.icalendar_instance.subcomponents[0]["SUMMARY"] = "foobar"
@@ -3701,7 +3701,7 @@ def testCreateOverwriteDeleteEvent(self):
 
         ## add same event again.  As it has same uid, it should be overwritten
         ## (but some calendars may throw a "409 Conflict")
-        if not self.check_compatibility_flag("no_overwrite"):
+        if self.is_supported("save-load.mutable"):
             e2 = c.add_event(ev1)
             if todo_ok:
                 t2 = c.add_todo(todo)
@@ -3747,7 +3747,7 @@ def testCreateOverwriteDeleteEvent(self):
         # Verify that we can't look it up, both by URL and by ID
         with pytest.raises(self._notFound()):
             c.event_by_url(e1.url)
-        if not self.check_compatibility_flag("no_overwrite"):
+        if self.is_supported("save-load.mutable"):
             with pytest.raises(self._notFound()):
                 c.event_by_url(e2.url)
         if not self.check_compatibility_flag("event_by_url_is_broken"):
@@ -3804,7 +3804,7 @@ def testDateSearchAndFreeBusy(self):
         ## (But events should not be immutable!  One should be able to change an event, push the changes
         ## out to all participants and all copies of the calendar, and let everyone know that it's a
         ## changed event and not a cancellation and a new event).
-        self.skip_on_compatibility_flag("no_overwrite")
+        self.skip_unless_support("save-load.mutable")
 
         # ev2 is same UID, but one year ahead.
         # The timestamp should change.