Calculate attributes when entity information available in Group sensor

[gjohansson-ST]

Proposed change

Change that attributes device class, state class and unit of measurement is only calculated once all entitiy can provide details (state or entity registry).

Fixes if group starts before integration of consumed entities or other related things when entity data is not available during group sensor starts.

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Additional information

• This PR fixes or closes issue: fixes Sensor group with sum does not consistently add unit #114835
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Stale

[catsmanac]

As noted in 118695 resolves the issue for the Group sums on utility_meter and integration but nested groups (Group sum of other Group Sum) still have the issue if one of the source groups get loaded after the group summing it.

[catsmanac]

Running updated code still has the issue for group sum summing other group sum. No UOM and history does not show as value trend.

[edenhaus]

Thanks @gjohansson-ST 👍

[edenhaus]

Merging it now as it solves the issues for most use cases