slack message template: invalid character '}' looking for beginning of object key string

i create a template for a message to slack:
```yaml
templates:
template.test: |
slack:
attachments: |
[{
"blocks": [
{
"type": "section",
"text": {
"type": "mrkdwn",
"text": "<{{.app.spec.source.repoURL}}/commit/{{.app.status.sync.revision}}|{{(call .repo.GetCommitMetadata .app.status.sync.revision).Message}}> by {{(call .repo.GetCommitMetadata .app.status.sync.revision).Author}} in {{ .context.environment }}",
}
}
]
}]
```
which is an attachment like:
```
{
"attachments": [
{
"blocks": [
{
"type": "section",
"text": {
"type": "mrkdwn",
"text": " by author in dev:cookie:"
}
}
]
}
]
}
```
and according to the slack [block kit builder](https://app.slack.com/block-kit-builder/T2NR19KSR#%7B%22attachments%22:%5B%7B%22blocks%22:%5B%7B%22type%22:%22section%22,%22text%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22%3Chttps://github.com/repo/commit/sha%7Cgithub%3E%20by%20author%20in%20dev:cookie:%22%7D%7D%5D%7D%5D%7D) this is ok.
but in the `argocd-notifications-controller` I get this error:
```
level=error msg="Failed to notify recipient {slack argocd-notifications} defined in resource argo-cd/keycloak: failed to unmarshal attachments '[{\n \"blocks\": [\n {\n \"type\": \"section\",\n \"text\": {\n \"type\": \"mrkdwn\",\n \"text\": \" by author in dev:cookie:\",\n } \n }\n ]\n}]\n' : invalid character '\\n' in string literal" resource=argo-cd/keycloak
```

Start at the argocd-notifications-controller path that unmarshals Slack attachments and reproduce the failure with the YAML template shown in the issue. Compare the controller's accepted attachment payload with the valid Block Kit example, then verify that the template produces a notification without an unmarshalling error.

Written by the indexing model from the issue text.