read_file decodes with UTF-8. Binary uploads (.xlsx, images, ...) raise
UnicodeDecodeError in the sandbox layer. UnicodeDecodeError is a ValueError
subclass, not an OSError, so it bypassed the typed handlers and fell through
to the generic except, surfacing a vague "Unexpected error reading file"
message. The model could not tell the file was binary, so it retried
read_file instead of switching to bash + pandas/openpyxl, burning LLM
round-trips and bloating context with repeated failures.
Add a dedicated UnicodeDecodeError handler that tells the model the file is
binary and to use bash with a suitable library (or view_image for images).
