Provide a built-in option to strip only the bot’s own mention from inbound activity text #833

The current activity.mentions.stripText: true option removes all structured mentions from inbound activity text. Bot command handlers commonly need different behavior: remove the bot’s leading mention while preserving mentions of other users.

For example:

Input:  <at>GitHub</at> help <at>Alice</at>
Output: help <at>Alice</at>

Today this requires application code to identify the mention whose account ID matches activity.recipient.id and remove it manually:

for (const entity of activity.entities ?? []) {
  if (
    entity.type === 'mention' &&
    entity.mentioned?.id === activity.recipient.id &&
    entity.text
  ) {
    activity.text = activity.text.replace(entity.text, '').trim();
    break;
  }
}

The lower-level stripMentionsText utility supports filtering by accountId, but applications must know to call it and pass the recipient ID for every activity. The app-level mention middleware could provide this common behavior directly because it already has access to the activity and its recipient.

Would you consider an option such as:

const app = new App({
  activity: {
    mentions: {
      stripText: {
        recipient: true,
        leadingOnly: true,
      },
    },
  },
});

or a dedicated utility:

stripRecipientMentionText(activity, { leadingOnly: true });

Expected behavior:

• Remove the bot/recipient mention from the beginning of the message.
• Preserve mentions of other users.
• Preserve a bot mention appearing later in the message when leadingOnly is enabled.
• Handle mention entities that provide either text or only the mentioned account’s name.
• Trim whitespace left by removing the leading mention.

This would cover the common Teams bot command-parsing scenario without requiring each application to implement its own mention matching and text mutation.