MigrateFromJSON previously called AddFullMessage in a loop, then
renamed the .json file to .json.migrated. If the process crashed
after appending some messages but before the rename, a retry would
re-read the same .json and append all messages again — duplicating
whatever was written before the crash.
Switch to SetHistory which atomically replaces the session contents.
A retry after crash overwrites the partial data instead of appending.
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