Single-op replace/remove failed with a dead-end 'old_text is required'
error when a structured-output client omitted the optional old_text field
(it can't be schema-required without a top-level if/then combinator that
OpenAI's Codex backend 400s on). The model couldn't recover.
Now a missing old_text returns the current entry inventory plus a retry
instruction (mirroring the batch path's _batch_error), so the model can
reissue the call with old_text set. Also sharpens the old_text schema
description to state it's required for replace/remove.
Fixes#49466, #43412.
c6bf6bda90