switch_model() swapped model/provider/base_url/api_key but never
refreshed agent._credential_pool, which stays bound to the original
provider. recover_with_credential_pool() then sees a pool.provider !=
agent.provider mismatch and short-circuits — so a 429/401 on the new
provider gets no rotation and falls through to fallback instead.
Reload load_pool(new_provider) inside switch_model when the provider
changes (or the pool is missing). The reload is inside the protected
swap block and the pool is added to the rollback snapshot, so a failed
client rebuild restores the original pool.
Fixes#16678, #52727.
Teknium
Tranquil-Flow