Summary

• salvage PR fix(test): isolate codex provider tests from local env #1037 onto current main by cherry-picking the contributor's test-isolation fixes with authorship preserved
• prevent test_codex_provider_uses_config_model from hitting live Codex model discovery when local credentials are present
• clear config-backed model state in the incompatible-default-model Codex test so local user config cannot leak into _model_is_default
• keep the follow-up fixture shape needed to avoid the base_url KeyError on current main

Test plan

• python -m pytest tests/test_cli_provider_resolution.py -n0 -q
• python -m pytest tests/ -n0 -q

Salvages #1037