What Problem This Solves

The LINE action builders in extensions/line/src/actions.ts enforce LINE's
field limits by calling String.prototype.slice directly:

• messageAction / uriAction / postbackAction / datetimePickerAction →
  label.slice(0, 20)
• postbackAction / datetimePickerAction → data.slice(0, 300) and
  displayText?.slice(0, 300)

A raw UTF-16 slice cuts an emoji in half whenever the limit lands inside a
surrogate pair. The returned label then ends in an unpaired high
surrogate, which LINE renders as a replacement character (or rejects the
payload).

Repro: messageAction("1234567890123456789😀") — 19 ASCII chars + 😀
(U+1F600, 2 UTF-16 code units) = 21 code units.

The fix swaps the raw slices for truncateUtf16Safe(value, limit) from
openclaw/plugin-sdk/text-utility-runtime, which drops a dangling surrogate
instead of emitting it. This matches how this plugin already truncates LINE
template titles (truncateTemplateText in template-messages.ts). Normal
inputs (short labels, exactly-at-limit ASCII, fully-fitting emoji) are
unchanged, and displayText stays undefined when omitted.

Evidence

Standalone red→green proof (replicates truncateUtf16Safe + the old vs. new
action logic):

RED  ok: old messageAction label = "1234567890123456789\ud83d" (lone surrogate present)
GREEN ok: messageAction label = "1234567890123456789"
GREEN ok: postbackAction label/data/displayText all surrogate-safe
GREEN ok: postbackAction displayText undefined is preserved
GREEN ok: normal/unaffected inputs unchanged

ALL ASSERTIONS PASSED — fix produces expectedOutput and preserves normal behavior.

The script asserts:

• RED: the old label.slice(0, 20) keeps a lone high surrogate (\uD83D).
• GREEN: the fixed builders return "1234567890123456789" (no lone
  surrogate) for label, and surrogate-safe data / displayText.
• Preservation: "Yes" → "Yes", 20-char ASCII unchanged, 25-char ASCII
  truncated to 20, fully-fitting "hi😀" preserved, and displayText stays
  undefined when not passed.

New unit tests in extensions/line/src/message-cards.test.ts cover
messageAction, uriAction, postbackAction (label, data, displayText), and
datetimePickerAction, asserting no /[\uD800-\uDBFF](?![\uDC00-\uDFFF])/
lone surrogate survives truncation.

🤖 Generated with Claude Code