Photon (iMessage) has no real edit API for already-sent messages. When
streaming completes, the gateway attempts to edit the message to remove
the streaming cursor (▉). Without edit support, this cursor gets stuck
in the final message, corrupting Unicode characters.
This change sets SUPPORTS_MESSAGE_EDITING=False on PhotonAdapter, which
causes the gateway to suppress the streaming cursor entirely for this
platform (via _effective_cursor in gateway/run.py). This prevents the
stale tofu square (▉) from appearing in streamed iMessage responses.
Fixes#49253
