Fix duplicate template handling in Persistent Notifications

[frenck]

Proposed change

Fixes an issue with the persistent notification services. The service
schema handled each value as a template, however, this is already handled
by the services from config. Nevertheless, it is used directly in code as well.

This PR adjusts the service schema to only render templates, else expect a string.
In the create method we now check if we are dealing with a template, before we
render it.

As an added bonus, this reduces the amount of useless Template renders (for non-templates).

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example entry for configuration.yaml:

# Example configuration.yaml
alias: New Automation
description: ''
trigger:
  - platform: mqtt
    topic: z2m/family_room_scene_controller/91/0/scene/003
condition: []
action:
  - service: persistent_notification.create
    data:
      message: '{{ trigger.payload }}'
mode: single

Additional information

• This PR fixes or closes issue: fixes MQTT trigger.payload being interpreted as a dict when passed JSON #47193
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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• Tests have been added to verify that the new code works.

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[balloob]

I'm surprised this fixes it.

When a template is static, we already don't render it but just return it from async_render (or parse it to create a new object if parse_result=True).

Oh ha: this works because cv.string will extract the raw value again and so it's back to the old value.