feat(config): implement JsonConfigSource manipulate fallbacks

Replace the 10 fallback-closure todo!()s in manipulate_json with real
PhpMixed manipulation. The fallback rewrites the decoded composer.json
when the JsonManipulator clean update fails.

- manipulate_json: drop the args Vec and array_unshift_ref (the latter
  called the array_unshift shim and only existed to thread config through
  args). Fallbacks now take &mut config directly and capture typed values,
  mirroring the clean closures. fallback returns Result so insertRepository
  can throw on a missing reference repository.
- Add private helpers: normalize_repositories_to_list,
  dedupe_repositories_by_name, set_nested, unset_nested, is_auth_config_key.
- Faithfully mirror PHP quirks in the dormant fallback path, including the
  reset()+foreach double-walk of the first segment in add/removeProperty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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