Fix Codex model catalog being wiped by live-config backfill

`modelCatalog` is a cc-switch-private field whose SSOT is the database; Live's
config.toml only carries a lossy `model_catalog_json` projection. Proxy
takeover/restore cycles and the official Codex.app rewriting config.toml can
drop that projection, so `read_live_settings` reconstructs an empty catalog.
Two paths then overwrote the stored mapping with that empty Live snapshot:

- Switch-away backfill (`switch_normal` -> `restore_live_settings_for_provider_backfill`):
  now overlays the DB provider's `modelCatalog`, falling back to the
  Live-reconstructed one only when the DB has none.
- Edit dialog (`EditProviderDialog`): when editing the active Codex provider it
  preferred Live over the DB SSOT; now keeps the DB `modelCatalog` so opening +
  saving no longer clears the mapping table.

Add Rust backfill tests (preserve DB catalog when Live lacks it; keep Live
catalog when DB has none) and a frontend regression test for the edit dialog.