* fix(wizard): preserve existing default model during setup auth choice
Without preserveExistingDefaultModel: true, the setup wizard
overwrite the user's configured default model when a new provider
auth is selected. This causes existing heartbeat turns to silently
consume paid API quota (e.g. Google Gemini) instead of the user's
original model.
The configure.gateway-auth.ts path already passes this flag; the
setup wizard path was missing it.
Fixes#64129
* fix(wizard): add type assertion for preserveExistingDefaultModel test
b836946879