Two provider routing bugs that cause silent wrong-model failures:
1. model.ts: getUserSpecifiedModelSetting() read ANTHROPIC_MODEL ||
GEMINI_MODEL || OPENAI_MODEL with no provider check. A user
switching from Anthropic to OpenAI with ANTHROPIC_MODEL still set
would silently send the Anthropic model name to the OpenAI API.
Now gates each env var behind the active provider from
getAPIProvider().
2. providers.ts: isCodexModel() maintained a hardcoded list of 8 model
names that was missing gpt-5.4-mini and gpt-5.2 from the canonical
CODEX_ALIAS_MODELS table in providerConfig.ts. This caused a
split-brain: getAPIProvider() returned 'openai' while
resolveProviderRequest() selected 'codex_responses' transport.
Now delegates to the exported isCodexAlias() to keep both detection
systems in sync.
fbf3385395