When a user runs out of OpenRouter credits and switches to Codex (or any
other provider), auxiliary tasks (compression, vision, web_extract) would
still try OpenRouter first and fail with 402. Two fixes:
1. Payment fallback in call_llm(): When a resolved provider returns HTTP 402
or a credit-related error, automatically retry with the next available
provider in the auto-detection chain. Skips the depleted provider and
tries Nous → Custom → Codex → API-key providers.
2. Remove hardcoded OpenRouter fallback: The old code fell back specifically
to OpenRouter when auto/custom resolution returned no client. Now falls
back to the full auto-detection chain, which handles any available
provider — not just OpenRouter.
Also extracts _get_provider_chain() as a shared function (replaces inline
tuple in _resolve_auto and the new fallback), built at call time so test
patches on _try_* functions remain visible.
Adds 16 tests covering _is_payment_error(), _get_provider_chain(),
_try_payment_fallback(), and call_llm() integration with 402 retry.
da02a4e283