Add hvac modes back to opentherm

[balloob]

Description:

Add hvac_modes to opentherm. Empty as it does not support setting it?

Fixes #25239

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

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly. Update and include derived files by running python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[ghost]

Hey there @mvn23, mind taking a look at this pull request as its been labeled with a integration (opentherm_gw) you are listed as a codeowner for? Thanks!

This is a automatic comment generated by codeowners-mention to help ensure issues and pull requests are seen by the right people.