Alerting: Alert rule annotation/label template validation

### What happened? While trying to create a custom annotation using results of a query (https://grafana.com/docs/grafana/latest/alerting/alerting-rules/templates/) I kepr receiving the original string back with the template markers and everything (e.g. `{{ $values }}`) I created elements of the annotation by copying and pasting fragments from the documentation and adjusting them to my case. Example input string: ``` Disk space on one or more of filesystems on the host is low (under 20%). Path: {{ index $labels "mountpoint" }}. Free space: {{ $values.available }}% {{ $labels }} {{ $values }} {{ $value }} ``` Actual result in the alert notification: ``` Disk space on one or more of filesystems on the host is low (under 20%). Path: {{ index $labels "mountpoint" }}. Free space: {{ $values.available }}% {{ $labels }} {{ $values }} {{ $value }} ``` When checking the Grafana logs, I've noticed the following error: ``` Feb 06 17:59:35 monitoring grafana[8439]: logger=ngalert.state.manager rule_uid=eec96kpmduvi8d org_id=1 t=2025-02-06T17:59:35.00638563Z level=error msg="Error in expanding template" error="failed to expand template '{{- $labels := .Labels -}}{{- $values := .Values -}}{{- $value := .Value -}}Disk space on one or more of filesystems on the host is low (under 20%). Path: {{ index $labels \"mountpoint\" }}. Free space: {{ $values.available }}% {{ $labels }} {{ $values }} {{ $value }}': error parsing template __alert_Free disk space under 20% - foobar: template: __alert_Free disk space under 20%!f(parse.item={ 0 184 unrecognized character in action: 1})oobar:1: %!s(MISSING)" ``` and that's how I found out there was a Unicode (NBSP) character hidden inside the copied and modified string (found it with https://bobpritchett.com/unicode-inspector): ``` "Disk space on one or more of filesystems on the host is low (under 20%). Path: {{ index $labels \"mountpoint\"\u00a0}}. Free space: {{ $values.available }}% {{ $labels }} {{ $values }} {{ $value }}" ``` ### What …