Review follow-ups for the salvaged picker partitioning:
- Drop the _model_chunks stash — it went stale when navigating to a
provider with an empty model list (early return skipped the
reassignment), producing a wrong 'N more available' count. Derive
shown = min(len(models), 75) directly instead.
- Add _DISCORD_SELECT_MAX_OPTIONS / _DISCORD_SELECT_MAX_ROWS constants
per the file's named-limits convention; replaces 4 bare literals.
- Remove dead total_rows variable.
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