Add water_heater to google_assistant

[jbouwh]

Proposed change

This is a second attempt to add water_heater support to google_assistant. Started by @hookedonunix with #93648

Water heaters with google assistant support contains:

• On/Off
• Set temperature (not visible in UI, but you can ask the temperature or request a new one.
• Set operation mode (mode can be selected in the UI).

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: #Add water_heater support for google_assistant home-assistant.io#30384

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
• I have followed the perfect PR recommendations
• The code has been formatted using Ruff (ruff format homeassistant tests)
• Tests have been added to verify that the new code works.

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

• Documentation added/updated for www.home-assistant.io

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

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• For the updated dependencies - a link to the changelog, or at minimum a diff between library versions is added to the PR description.
• Untested files have been added to .coveragerc.

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

[home-assistant]

Hey there @home-assistant/cloud, mind taking a look at this pull request as it has been labeled with an integration (google_assistant) you are listed as a code owner for? Thanks!

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[emontnemery]

Thanks @jbouwh 👍

Some minor remarks on code style.

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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[emontnemery]

LGTM, thanks @jbouwh 👍

[jbouwh]

Seems I forgot to add water_heater to the default exposed domains. Added that with an extra commit.