fix(bot-mode): backfill follow-profile contract for legacy canonical Bot Chats

Bot Chats created before the follow_profile_config marker existed carry no
contract in model_config, so they would stay pinned to a stale stored
provider until deleted — the exact shape of the live reports (#89497,
#94818). Mirror the plugin's own identity rule (the profile's session
titled exactly 'Bot Chat') as a legacy fallback in
_stored_session_runtime_overrides, matching the room-plumbing legacy
'Group:' title fallback.

Follow-up to the salvaged #90343 (@curator8888) and #96111 (@lorzl).

contributors/emails/david@davidtyler.rocks

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+curator8888