Allow OpenUV entities to be unavailable

[bachya]

Description:

This PR adds the available property to all OpenUV entities so that they properly render as unavailable in Lovelace.

Related issue (if applicable): fixes #30973

Pull request with documentation for home-assistant.io (if applicable): N/A

Example entry for configuration.yaml (if applicable):

openuv:
  api_key: !secret openuv_api_key

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

[bachya]

Not sure why CI is failing. 🤷‍♂

[frenck]

@bachya We've seen that a couple of time before today 😢

It is fixed by adding an empty commit.
git commit --allow-empty -m "Empty commit to re-trigger build"