Handle unknown search result kinds without throwing #226

[JbravoI]

Summary

Fixes an edge case in issue #226 where the search response converter throws NotImplementedException when Companies House introduces an unrecognised search result kind.

Changes

• Added UnknownSearchItem as a generic fallback model.
• Updated SearchItemConverter to deserialize unknown kind values into UnknownSearchItem.
• Preserves the raw kind value for callers.
• Added a regression test covering an unknown search result kind.

Why

Companies House may add new search result types over time. Deserialization should remain forward-compatible instead of failing the entire response when an unfamiliar kind is returned.

Testing

Added scenario test:

SearchAllPayload_DeserializesUnknownKindAsGenericSearchItem

The test verifies that:

• an unknown search result kind does not throw
• the item is deserialized as UnknownSearchItem
• the original kind value is preserved

[kevbite]

@JbravoI thanks for the contribution, I'll get this merged in!