The #17929 init-time fallback block was nested inside the
'_explicit not in {auto, openrouter, custom}' guard, so an exhausted
openrouter credential pool skipped fallback_providers entirely and
AIAgent.__init__ raised 'No LLM provider configured' — surfacing on
Telegram only as the generic 'unexpected error' message.
2026-08-23 outage (~22:09-23:50, 60 occurrences incl. a cron job):
single-entry openrouter pool hit daily free-tier quota; chain had a
healthy local Ollama entry that never got tried because provider was
the default 'openrouter'.
Un-nest the block so any primary without usable credentials walks the
chain before failing; providers explicitly chosen by name keep their
dedicated missing-key diagnostic when both primary AND chain fail.
Regression tests cover the openrouter-exhausted path both with and
without a usable chain entry.
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
VJ Pixel