Sliding sync: name field should fall back to canonical alias if there is no room name

[erikjohnston]

Alternatively, we could add canonical_alias as a separate field.

This is breaking the names of rooms without a m.room.name in EX. Apparently the rust SDK doesn't use name field.

c.f.

synapse/synapse/handlers/sliding_sync/__init__.py

Lines 1013 to 1019 in f6c2b0e

	room_name: Optional[str] = None
	# TODO: Should we also check for `EventTypes.CanonicalAlias`
	# (`m.room.canonical_alias`) as a fallback for the room name? see
	# https://github.com/matrix-org/matrix-spec-proposals/pull/3575#discussion_r1671260153
	name_event = room_state.get((EventTypes.Name, ""))
	if name_event is not None:
	room_name = name_event.content.get("name")

[MadLittleMods]

Relevant MSC discussion: matrix-org/matrix-spec-proposals#3575 (comment)

[MadLittleMods]

The name field is strictly just the m.room.name value right now. The MSC discussion asks about whether we should fallback to m.room.canonical_alias which aligns with what the spec suggests for whether heroes should be sent in Sync v2.

Another alternative is that clients can simply just request m.room.canonical_alias in the required_state. This is what the Rust SDK is already doing -> matrix-org/matrix-rust-sdk#3982