atomic_yaml_write used default yaml.dump which emits indentless
sequences (list items at column 0), while atomic_roundtrip_yaml_update
(ruamel.yaml) emits 2-space-indented sequences. Cross-path writes to
the same config.yaml toggled indentation on every save, eventually
producing a mixed-indent file that js-yaml rejects with 'bad indentation
of a mapping entry', silently dropping custom_providers and breaking
model switching.
Add IndentDumper SafeDumper subclass that forces indentless=False,
route atomic_yaml_write through it. Route tui_gateway._save_cfg and
the Telegram adapter's config writer through atomic_yaml_write so all
paths emit the same 2-indent layout.
Salvaged from #32034 by @xxxigm. Adapted to current main which already
has allow_unicode=True (from #51356) but was missing IndentDumper.
Closes#31999