Remove display-only media placeholders from user text.
Placeholders like [image 1] are inserted into the terminal input purely for
display; the actual media travels as structured content blocks. They must not
leak into the canonical model-facing message or LangSmith trace as if the user
typed them.
When available, tracked placeholder spans identify the exact display tokens to strip so user-authored literal duplicates with the same token are preserved. The token fallback removes one matching occurrence per tracked media item for callers that only have placeholder text.
Exact (start, end) spans for tracked display tokens
in text, when known.
