fix: adjust HASS state classes for rain entities

[bachya]

Describe what the PR does:

This is based on the suggestion from the HASS docs.

Does this fix a specific issue?

Fixes https://github.com/bachya/ecowitt2mqtt/issues/1009

Checklist:

• [ ] Confirm that one or more new tests are written for the new functionality.
• [x] Run tests and ensure everything passes (with 100% test coverage).
• [ ] Update README.md with any new documentation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (58b0d24) to head (07912a8).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1018 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 32 32 Lines 1775 1775 ========================================= Hits 1775 1775 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.