Add explicit gemini_oauth handling in create_cheap_provider_for_backend()
to create a GeminiOauthProvider with the cheap model swapped in. Without
this, setting LLM_CHEAP_MODEL with gemini_oauth backend would fail with
a confusing "no registry provider config available" error.
Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>