fix(ui): rename model config files on save to prevent duplicates (#9388)

Editing a model's YAML and changing the `name:` field previously wrote
the new body to the original `<oldName>.yaml`. On reload the config
loader indexed that file under the new name while the old key
lingered in memory, producing two entries in the system UI that
shared a single underlying file — deleting either removed both.

Detect the rename in EditModelEndpoint and rename the on-disk
`<name>.yaml` and `._gallery_<name>.yaml` to match, drop the stale
in-memory key before the reload, and redirect the editor URL in the
React UI so it tracks the new name. Reject conflicts (409) and names
containing path separators (400).

Fixes #9294