Truncating Telegram HTML/Markdown alerts returns 400

Up to recently I thought that Grafana did not truncate messages to 4096 and that was the reason I kept getting errors 400 from Telegram API (in the logs). When considering doing a PR I found https://github.com/grafana/alerting/blob/main/receivers/telegram/telegram.go#L120 and tested simpler alerts and indeed it was truncated and alert sent successfully. Then I found somewhere that Telegram requires messages with Markdown/HTML parse_mode to be *valid* and I finally managed to find my issue: truncating often removes a terminating tag (as I use HTML) leaving the opening tag, resulting in invalid HTML. I've reproduced this easily using `Contact points` test message. I just filled a message `<b>a</b>` preceeded by 4089 `A` and clicking `Test` results in error 400. Error 400s are easily confirmed by just using `<b>a` as message or `_a` with Markdown parse mode. I usually like to provide possible solutions yet I don't have an easy one in this case 😞 I believe the proper way would be to, after truncating, to loosely parse the message (in the proper parse_mode) and close any blocks that are missing, yet "sounds heavy"... A simpler workaround could be to a new feature which is to use allow a `Contact Point` to have "fallback" integrations. Currently one with multiple integrations will always notify all integrations. If there was a "fallback" integration, it would mean it's only notified if a default one fails. This would allow to set up a contact point with 2 Telegram integrations: one with pretty markdown/html template and a second with a simpler template and parse_mode `None`.