Previously, migrateConfigFile() mutated rawConfig directly. If the file
write failed (e.g. read-only file, permissions), the in-memory config was
already changed to the migrated values, causing the plugin to use migrated
models even though the user's file was untouched. On the next run, the
migration would fire again since _migrations was never persisted.
Now all mutations happen on a structuredClone copy. The original rawConfig
is only updated after the file write succeeds. If the write fails,
rawConfig stays untouched and the function returns false.
