Warn on non-string config entry unique IDs

[epenet]

Breaking change

Config entries should have unique_id as None or as a string.
Non-string unique_id should be amended/migrated to use a string.

Proposed change

Follow-up to #109966
Spotted in #125313

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 #
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[emontnemery]

Looks good, just a minor nit.