adapter-teams: outgoing @mentions render as literal <at> tags (no mention entities; multi-word names split)

[vody]

Summary

Two related defects in @chat-adapter/teams / @chat-adapter/shared (observed on 4.37.0) make bot-authored @Name Surname mentions render as the literal text <at>Name</at> Surname in Microsoft Teams instead of a mention chip.

1. No mention entities are attached on send

TeamsFormatConverter.convertMentionsToTeams rewrites bare @name to <at>name</at> in outgoing text (dist/index.js ~1247):

convertMentionsToTeams(text) {
  return replaceBareMentions(text, (_mention, name) => `<at>${name}</at>`);
}

But Teams only renders <at> as a mention when the activity also carries a matching entity:

"entities": [{ "type": "mention",
               "text": "<at>Name Surname</at>",
               "mentioned": { "id": "29:...", "name": "Name Surname" } }]

The adapter never constructs outgoing mention entities (it reads them on inbound — the e.type === "mention" check around dist/index.js ~1837 — but no send path writes them). Without the entity, Teams displays the raw tag as literal text, which is worse than not converting at all.

2. replaceBareMentions captures a single word

In @chat-adapter/shared (dist/index.js ~412), the mention scanner consumes @ plus one word-character run:

if (text[index] === "@" && ... && isWord(text[index + 1])) {
  let cursor = index + 2;
  while (cursor < end && isWord(text[cursor])) cursor += 1;
  const mention = text.slice(index, cursor);
  ...
}

@Hank Feng therefore becomes <at>Hank</at> Feng — even with entities fixed, the chip would carry half the display name and fail to match the roster name.

Observed impact (production)

A bot reply beginning Hi @Hank Feng — ... was delivered to a Teams channel as the literal text Hi <at>Hank</at> Feng — ....

Expected behavior

• Either resolve outgoing @mentions against the conversation roster and attach mention entities (matching the full display name greedily), or leave @name untouched / strip the @ when no entity can be built. Emitting <at> without an entity should never happen.
• Mention capture should support multi-word display names (e.g. greedy longest-match against known conversation members).