truncateText sliced the approval card text paragraph with String.slice,
which can cut through an astral character's surrogate pair (e.g. an emoji
straddling the 1797-char limit), leaving a lone surrogate in the card
text sent to Google Chat.
Use truncateUtf16Safe from the plugin SDK so truncation never splits a
surrogate pair, keeping the '...' suffix and the existing length budget.
Adds tests asserting the truncated Command card text stays UTF-16 well
formed and that an astral character is preserved when it fits.
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