Dashboard variables_v2 wire shape

Trigger: Editing variables_v2 expand/flatten (variables_v2.go) or static values[].label schema/plan modifier (dashboard_schema/variables_v2.go).

Fix: Keep Terraform UX flat, then build typed SDK models (not map[string]any JSON):

• value arms → SingleStringValue{Value: &StringValueLabel{...}} (same for regex/lucene/interval/numeric)
• id string ↔ UUID{Value: ...} via ExpandDashboardUUID
• promql query string ↔ PromQlQuery{Value: ...} via ExpandPromqlQuery
• dataprime query string ↔ CommonDataprimeQuery{Text: ...}
• observation scope short name ↔ DATASET_SCOPE_* via existing enum maps
• spans value = { type, value } ↔ SpanField via ExpandSpansField / FlattenSpansField
• metrics operator {type, selected_values} ↔ {equals|notEquals: {selection: {list: ...}}}
• empty metrics label_filters → omit/nil (avoid null↔[] drift)
• empty value_display_options: schema AtLeastOneOfChildren rejects {} (Framework cannot plan null when config keeps the block; omit the block or set a regex)
• empty multi_string.list.values → flatten known empty list, not null (values is Required)
• enums (display_type, order, refresh, data_mode) → OptionalEnumPointer (never pointer-to-empty)
• OpenAPI-required fields (VariableV2 id/name/display_name/display_type/source/value, static/query all_option + values_order_direction, ValueLabel value/label, AllOption.include_all, logs FieldValue.observation_field, metrics LabelValue.label_name) are non-pointer SDK values — expand with requiredEnumValue / derefOrZero / ValueString(), flatten by address (&variable.Source) and types.StringValue / types.BoolValue. TF schema stays Optional+Computed+Default where already defaulted; do not re-mark schema Required just because the Go type lost *.
• static values[].label: Optional+Computed + StaticValueLabelFromValue (omit → plan=value). Flatten always stores API label (incl. label==value). Never UseStateForUnknown — needed for omit→set→omit and omit→value-change. Expand still copies value if label null/unknown. On flatten, fill missing oneof siblings as null from schema AttrTypes (API omits unset arms).

Why: OpenAPI types nest wrappers and proto enums; flat TF fields must be wrapped into typed SDK structs or the API returns 400 / ObjectValueFrom fails. Label defaults via plan (not flatten-to-null) so omit, custom, explicit label=value, and value changes all round-trip without drift.