``InlineKeyboardButton(label, callback_data=label)`` fails Telegram's
API when the label exceeds 64 bytes UTF-8. An LLM-generated long
option (realistic in multilingual flows) used to 400 the ``send_message``
call silently — user got nothing, agent heard a successful retry-then-drop.
Decouple display from wire: button text keeps the full label, callback_data
gets truncated at a UTF-8 char boundary. Tap echoes the prefix back as the
user message; the LLM understands a prefix of its own option just fine,
and the display the user saw was always the full string.
Locks: helper boundary behavior (ASCII, CJK, short labels pass through)
and end-to-end ``_build_keyboard`` integration with an over-cap label.
Made-with: Cursor