When a provider's credential pool has a single entry in 429-cooldown,
resolve_provider_client returns None and AIAgent.__init__ raises a
misleading RuntimeError suggesting the API key is missing — even when
valid fallback_providers are configured.
This patch makes __init__ iterate the fallback chain before raising,
mirroring the existing in-flight fallback logic in the request loop.
If a fallback resolves, the agent initializes against it and sets
_fallback_activated=True so _restore_primary_runtime can pick the
primary back up after cooldown.
Closes#17929
