getComboVisionBridgeDecision() treated any top-level combo-ref step as an
unconditional "process", without ever resolving the referenced combo's real
leaf models. A pass-through combo whose only member is a combo-ref to an
all-vision-capable inner combo was wrongly routed through the
describe-and-replace path, and with no describer model configured every image
was replaced with the literal stub text.
Recursively resolve combo-ref steps to their real leaf models (depth-guarded
by the same MAX_COMBO_DEPTH used by the flatten dispatch path, plus a
visited-set cycle guard) and fold their vision capability into the same
accumulation used for direct model steps. An unresolvable combo-ref (not
found / empty / circular / depth-exceeded) is conservatively treated as a
single non-vision-capable leaf instead of forcing the whole combo to
"process".
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