make group_by optional in the groups embeddable schema

[mgiota]

Summary

Issue 1

The group_filters object is itself optional (schema.maybe(...)). Inside it, groups, filters, and kql_query are all optional too. But group_by was required once you provided group_filters. That means if you wanted to create a group SLO embeddable via the API with just a KQL filter:

{
"group_filters": {
"kql_query": "slo.name: "my-slo""
},
"overview_mode": "groups"
}

...it would fail validation because group_by is missing, even though both the UI and the transform layer already
default it to 'status' when absent. Making group_by optional in the schema aligns validation with the actual semantics: 'status' is the default, and the consumer code already handles the absent case.

Issue 2
The schema uses schema.oneOf([single, groups]) from @kbn/config-schema. Its UnionType.handleError always reports errors from ALL variants, producing misleading messages like [0.slo_id]: expected value of type [string] but got [undefined] even in groups mode.

Acceptance criteria

• Make group_by optional in GroupOverviewCustomSchema
• Fix useState default precedence in slo_group_filters.tsx
• Default group_by to 'status' in transform_group_overview_out.ts
• Switch schema.oneOf to schema.discriminatedUnion('overview_mode', ...) in getOverviewEmbeddableSchema
• Adapt OAS tests