Use room_version capability for state-DAG checks

Replace `isinstance(event, FrozenEventVMSC4242)` with
`event.room_version.msc4242_state_dags` in the two places where the
class check was used as a state-DAG capability check. This is the more
correct mental model — the relevant question is whether the room version
uses state DAGs, not which Python subclass we happen to have constructed
— and it lets the next commit drop `FrozenEventVMSC4242` along with the
rest of the class hierarchy.

A `cast(FrozenEventVMSC4242, event)` keeps mypy happy until the Rust
`Event` is wired in and gains a real `prev_state_events` getter.

synapse/handlers/message.py

@@ -22,7 +22,7 @@
 import logging
 import random
 from http import HTTPStatus
-from typing import TYPE_CHECKING, Any, Mapping, Optional, Sequence
+from typing import TYPE_CHECKING, Any, Mapping, Optional, Sequence, cast
 
 from canonicaljson import encode_canonical_json
 
@@ -1871,8 +1871,8 @@ class EventCreationHandler:
 
             state_entry = await self.state.resolve_state_groups_for_events(
                 event.room_id,
-                event_ids=event.prev_state_events
-                if isinstance(event, FrozenEventVMSC4242)
+                event_ids=cast(FrozenEventVMSC4242, event).prev_state_events
+                if event.room_version.msc4242_state_dags
                 else event.prev_event_ids(),
             )
 

synapse/state/__init__.py

@@ -29,6 +29,7 @@ from typing import (
     Mapping,
     Optional,
     Sequence,
+    cast,
 )
 
 import attr
@@ -314,8 +315,8 @@ class StateHandler:
             # their states - but I don't think that really matters; it just means we
             # might redundantly recalculate the state for this event later.)
             prev_event_ids = frozenset(
-                event.prev_state_events
-                if isinstance(event, FrozenEventVMSC4242)
+                cast(FrozenEventVMSC4242, event).prev_state_events
+                if event.room_version.msc4242_state_dags
                 else event.prev_event_ids()
             )
             incomplete_prev_events = await self.store.get_partial_state_events(