PR #1827 introduced _read_visible_codex_cache_model_ids() merging
into the providers card live-fetch path. The two v0.51.19 tests in
tests/test_issue1807_codex_provider_card_live_models.py predate that
helper and didn't isolate CODEX_HOME, so the dev machine's real
~/.codex/models_cache.json (which contains entries like
gpt-5.3-codex-spark from #1680) was leaking into their assertions.
Add CODEX_HOME isolation in the existing _configure_codex helper —
matches the pattern PR #1827's own test already uses. Test-only fix;
production code unchanged. Caught by pre-release pytest gate.
