What Problem This Solves

truncateSummary in extensions/feishu/src/streaming-card.ts builds the
closeout summary that is embedded in the Card Kit settings PATCH (config.summary.content).
It truncated with raw UTF-16 indexing:

return clean.length <= max ? clean : clean.slice(0, max - 3) + "...";

When an emoji or other astral-plane character (a UTF-16 surrogate pair)
straddles the max - 3 cut point, slice keeps the high surrogate and
drops the low surrogate, leaving a lone surrogate half in the summary.
Feishu renders that dangling half as the replacement character �.

Before / after (repro): input is "a".repeat(46) + "😀" + "b".repeat(20)
(a 68-char string with 😀 / U+1F600 at UTF-16 indices 46–47), default max = 50,
so the slice happens at max - 3 = 47 — right between the two halves of the emoji.

• Before: "aaaa…(46 a's)…" + "\uD83D" + "..." — the summary ends with a lone
  high surrogate \uD83D before ..., which serializes invalid and renders as �.
• After: "a".repeat(46) + "..." — the half-emoji is dropped whole.

The fix swaps the raw slice for the repo's surrogate-safe helper,
sliceUtf16Safe(clean, 0, max - 3) (from openclaw/plugin-sdk/text-utility-runtime),
which snaps the boundary off a surrogate pair so the straddling code point is
dropped whole. This mirrors the existing pattern in
extensions/slack/src/truncate.ts. ASCII / non-straddling inputs are unchanged.

Evidence

Node red-green proof replicating the exact sliceUtf16Safe helper plus the
OLD (buggy) and NEW (fixed) truncateSummary logic:

repro.length = 68
OLD output endsWith lone high surrogate \uD83D: true
OLD output JSON: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\ud83d..."
NEW output JSON: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa..."
PASS: OLD leaves a dangling high surrogate (bug present)
PASS: OLD does NOT equal expected (bug present)
PASS: NEW equals expected (half-emoji dropped whole)
PASS: NEW contains NO lone high surrogate
PASS: NEW is valid (no replacement char on encode roundtrip)
PASS: normal short unchanged
PASS: normal long ASCII unchanged vs old
PASS: normal long ASCII is 47 chars + ...
PASS: empty unchanged
PASS: non-straddling emoji preserved

ALL ASSERTIONS PASSED

• RED: the old slice(0, 47) output is "…\uD83D..." — a dangling high
  surrogate (charCodeAt(len-4) === 0xD83D), and is not equal to the expected
  surrogate-clean summary.
• GREEN: the fixed output equals "a".repeat(46) + "...", contains no lone
  surrogate, and survives a UTF-8 encode roundtrip without a �.
• No behavior change for unaffected inputs: short strings, long ASCII
  (still 47 chars + "..."), empty strings, and non-straddling emoji are all
  byte-identical to the previous behavior.

A unit test was added in extensions/feishu/src/streaming-card.test.ts
("drops a surrogate pair whole when truncating the closeout summary") that
drives session.close(finalText), captures the /settings PATCH body, and
asserts config.summary.content is "a".repeat(46) + "..." with no \uD83D.
It fails on the old code and passes after the fix.