* fix(openai-codex): honor providerConfig.baseUrl in dynamic-model synthesis fallback
The synthesis fallback in resolveCodexForwardCompatModel hardcoded
OPENAI_CODEX_BASE_URL when the model registry had no template row to
clone, which meant openai-codex providers configured with a custom
baseUrl (e.g. a local proxy that forwards Codex traffic) silently
fell back to api.openai.com / chatgpt.com - bypassing the proxy and
typically failing the auth contract.
Synthesis now reads ctx.providerConfig.baseUrl when present, with the
existing OPENAI_CODEX_BASE_URL constant as the fallback. No effect on
template-clone or registry-find paths, which already inherit the
configured baseUrl through the cloned template.
* docs(changelog): add Unreleased Fixes entry for #76428 codex synthesis baseUrl honor
b83b2e3f1c