fix(foreground-fallback): match bailian quota-exhausted error and prevent backward chain fallback

Adds /\bquota\b.*\bexhausted/i to RETRYABLE_ERROR_PATTERNS so bailian's
'Your token-plan 1-week quota has been exhausted' triggers failover.

Also seeds SessionTried with all chain entries at or before the current
model's index during execFallback, preventing re-fallback onto the dead
chain[0] when a downstream model has already been skipped.

Fixes #1083