Tool results and evidence can end on a lone surrogate half when clipped #207

What happened?

Mail and page text reaches tool results, recorded evidence and notifications through String.prototype.slice(0, limit) on the UTF-16 code units (for example the mail snippet at apps/server/src/engine/conversation.ts (snippet: body.slice(0, 240), read_mail_thread's body.slice(0, 12000)), the search/web/page clips in apps/server/src/engine/model.ts and the monitor/evidence clips in apps/server/src/engine/service.ts, plus browser.ts and jev/service.ts). When the cut index lands between the two halves of a surrogate pair (an astral character such as an emoji), the clipped value ends on a lone high surrogate:

• it is not a character: UI renderers show U+FFFD in its place, and
• consumers that encode the value to UTF-8 (for example any Python service that reads the same data) raise UnicodeEncodeError on the lone half.

Two places in the codebase already guard against exactly this: clipCalendarText in conversation.ts (added with the calendar tool) and sanitizeFileName in files.ts ("Slice code points, not UTF-16 units, so a surrogate pair is never split in half"), and #183 fixed the related case of a surrogate numeric character reference failing the mail list. The mail and page clips are the same class but do not use the guard.

Steps to reproduce

1. Give a mail body or watched page an astral character (for example 👍, U+1F44D, the pair D83D DC4D) straddling index 240 (or 12000/30000/500 for the other surfaces) — or in code: "a".repeat(239) + "\u{1F44D}".repeat(3).
2. const snippet = body.slice(0, 240) → the result ends with the lone half \uD83D: /[\uD800-\uDBFF]$/.test(snippet) is true.
3. The same value passed through JSON.stringify keeps a \ud83d escape, and encoding it to UTF-8 outside JS replaces or rejects the lone half.

Expected: clip at the same limit, but drop the high half with its low half when the cut would split a pair, the way clipCalendarText already does for calendar text.

Commit or release

main @ a4944f4.

Platform

Any (server-side string handling).

Environment and safe logs

Pure string handling; no provider or platform dependency. Reproduced with Node: ("a".repeat(239) + "\u{1F44D}").slice(0, 240).charCodeAt(239).toString(16) → d83d.