Layered fallback for auxiliary tasks (compression, vision, tts, web_extract,
session_search, etc.):
1. Primary aux provider (existing)
2. User-configured auxiliary.<task>.fallback_chain (new)
3. Main agent provider + model (new — last-resort safety net)
4. Warn user + re-raise original error (new)
For users on 'auto' (no explicit aux provider), the existing
_try_payment_fallback auto-detection chain runs instead — its Step 1
already IS the main agent model, so they get the same behaviour without
configuration.
The configured fallback_chain config schema comes from #26882 / @zccyman;
the main-agent safety net + exhaustion warning were added on top.
Closes#26882. Builds on the capacity-error gate fix in the previous
commit (#26803 / @Bartok9).
zccyman
a574246837