Slack: inbound `<mailto:…|label>` and `<tel:…|label>` links are not converted, so the label ends up inside the link URL

Bug Description

slackMrkdwnToMarkdown converts Slack link tokens only when the target starts with http:// or https://. Slack uses the same <target|label> token for mailto: and tel: links, and it auto-links every email address a user types, so ordinary messages contain <mailto:a@b.com|a@b.com>.

The token passes through the converter unchanged. parseMarkdown then reads <mailto:a@b.com|a@b.com> as a CommonMark autolink, which produces a link node whose url is mailto:a@b.com|a@b.com. message.text contains mailto:a@b.com|a@b.com, and stringifyMarkdown(message.formatted) emits the raw Slack token.

Steps to Reproduce

1. In Slack, send the bot a message that contains an email address, for example mail a@b.com please. Slack delivers text: "mail <mailto:a@b.com|a@b.com> please".
2. Read message.text and stringifyMarkdown(message.formatted) in the handler, or run the code sample below, which calls the same converter the adapter uses.

Expected Behavior

A link node with url: "mailto:a@b.com" and text a@b.com, the same treatment <https://…|label> gets. message.text is mail a@b.com please. The same applies to <tel:+15551234567|555-123-4567> and to the unlabelled forms <mailto:a@b.com> and <tel:+15551234567>.

Actual Behavior

Input (event.text)	link.url	message.text	stringifyMarkdown
mail <mailto:a@b.com|a@b.com> please	mailto:a@b.com|a@b.com	mail mailto:a@b.com|a@b.com please	mail <mailto:a@b.com|a@b.com> please
call <tel:+15551234567|555-123-4567> now	tel:+15551234567|555-123-4567	call tel:+15551234567|555-123-4567 now	call <tel:+15551234567|555-123-4567> now
mail <mailto:a@b.com> please	mailto:a@b.com	mail mailto:a@b.com please	mail <mailto:a@b.com> please

In the unlabelled case the URL is correct, but the visible text and message.text include the mailto: scheme.

Code Sample

import { SlackFormatConverter } from "@chat-adapter/slack";
import { stringifyMarkdown, toPlainText } from "chat";

const ast = new SlackFormatConverter().toAst("mail <mailto:a@b.com|a@b.com> please");
console.log(toPlainText(ast));        // "mail mailto:a@b.com|a@b.com please"
console.log(stringifyMarkdown(ast));  // "mail <mailto:a@b.com|a@b.com> please\n"

Chat SDK Version

4.30.0

Node.js Version

22.21.1

Platform Adapter

Microsoft Teams, Slack

Operating System

Linux

Additional Context

Workaround we run, applied to the mrkdwn before the converter in a SlackFormatConverter subclass:

const LABELLED_ADDRESS_LINK = /<((?:mailto|tel):[^|<>]+)\|([^<>]+)>/g;
const BARE_ADDRESS_LINK = /<((?:mailto|tel):([^|<>]+))>/g;

export function normalizeSlackMrkdwn(mrkdwn: string): string {
  return mrkdwn.replace(LABELLED_ADDRESS_LINK, "[$2]($1)").replace(BARE_ADDRESS_LINK, "[$2]($1)");
}

export class NormalizingSlackFormatConverter extends SlackFormatConverter {
  override toAst(mrkdwn: string): Root {
    return super.toAst(normalizeSlackMrkdwn(mrkdwn));
  }
}

Start at SlackFormatConverter.toAst, which currently handles Slack link tokens only for HTTP targets. Reproduce the labelled and unlabelled mailto: and tel: examples from the issue, then verify that the resulting link URL, visible text, message.text, and stringifyMarkdown output match the expected behavior.