Plex websocket callback async cleanup

[jjlawren]

Proposed change

Cleanup async use on new callback.

Followup from post-merge comments on #40773.

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

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

The integration reached or maintains the following Integration Quality Scale:

• No score or internal
• 🥈 Silver
• 🥇 Gold
• 🏆 Platinum

To help with the load of incoming pull requests:

• I have reviewed two other open pull requests in this repository.

[balloob]

This method is called by the Plex media server. This decorator doesn't do anything.

I think that the current code is correct?

[MartinHjelmare]

The websocket is running in async context so we need to change the api below. The decorator here is not needed but can be used for clarity.

[jjlawren]

I'm using the decorator more as a comment on the expected context.