Cast the volume_level of a universal media_player to a float

[michaelarnauts]

Description:

The universal media_player can be configured to fetch it's volume from another sensor's state (like from a mqtt source). Since a sensor state is string, the result of the media_player's volume_level will also be a string.

This causes issues in other parts of the code (like the google_assistant trait that returns the volume) that expect a volume_level to be a float instead of a string.

Config like this:

media_player:
- platform: universal
  name: receiver
  children:
    - media_player.kodi
    - media_player.living_room_chromecast
  commands:
    # ...
  attributes:
    # ...
    volume_level: sensor.avr_volume

sensors:
- platform: mqtt
  name: avr_volume
  state_topic: "media/cec/volume"
  availability_topic: "media/bridge/status"
  value_template: "{{ value_json / 100 | float }}"

Related issue (if applicable): fixes #29044

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]

[michaelarnauts]

Hmm, it seems that the tests assume the output is a string, but the rest Home Assistant doesn't. I assume the tests are wrong.