apply defaultValue to overview_mode for both single and groups cases

[mgiota]

Fixes elastic#255681

Summary

When creating a dashboard via POST kbn:/api/dashboards?apiVersion=1 with an SLO_EMBEDDABLE panel whose config is omitted overview_mode, validation fails with:

[request body.panels.0.0.config]: "overview_mode" property is required

• When slo_id is provided (or optionally an slo_instance_id as well), then defaultValue is single
• When no slo_id is provided and no overview_mode, then defaultValue is groups

Acceptance criteria

• when an slo_id is provided, but no overview_mode, then defaultValue is single
• when no overview_mode and no slo_id is provided, then defaultValue is groups

slo_id Provided	overview_mode Provided	defaultValue
Yes	No	single
No	No	groups

Approach

Switched from schema.discriminatedUnion to schema.oneOf so each branch can have its own defaultValue for overview_mode.

• getOverviewEmbeddableSchema() returns schema.oneOf([singleSchema, groupSchema]) with single branch first, then group.
• Single branch: overview_mode: schema.oneOf([literal('single'), literal('groups')], { defaultValue: 'single' }).
• Group branch: overview_mode: schema.oneOf([literal('single'), literal('groups')], { defaultValue: 'groups' }).
• When overview_mode is omitted: oneOf tries single first (defaults to 'single'); if slo_id is missing that fails, then group branch is used (defaults to 'groups'). So groups is the default when omitted unless slo_id is provided.

Trade off

Trade off is that a request like this with invalid group_by will report all failures from all branches.

POST kbn:/api/dashboards?apiVersion=1
{
  "title": "SLO group embeddable invalid group filters",
  "panels": [
    {
      "config": {
        "group_filters": {
            "group_by": "invalid group by"
        }
      },
      "grid": {
        "x": 1,
        "y": 0
      },
      "type": "SLO_EMBEDDABLE"
    }
  ]
}

{
  "statusCode": 400,
  "error": "Bad Request",
  "message": """[request body.panels.0]: types that failed validation:
- [request body.panels.0.0.config]: types that failed validation:
 - [request body.panels.0.config.0.slo_id]: expected value of type [string] but got [undefined]
 - [request body.panels.0.config.1.group_filters.group_by]: types that failed validation:
  - [request body.panels.0.config.group_filters.group_by.0]: expected value to equal [slo.tags]
  - [request body.panels.0.config.group_filters.group_by.1]: expected value to equal [status]
  - [request body.panels.0.config.group_filters.group_by.2]: expected value to equal [slo.indicator.type]
- [request body.panels.0.1.title]: expected value of type [string] but got [undefined]"""
}