[schema-coverage] Add acceptance coverage for elasticstack_elasticsearch_ingest_processor_split

[github-actions]

Entity: elasticstack_elasticsearch_ingest_processor_split\nEntity type: data source\nImplementation directory: internal/elasticsearch/ingest\n\n## Attributes with no coverage\n- id (Computed, TypeString) — not asserted directly.\n- target_field (Optional, TypeString) — not configured, not asserted.\n- description (Optional, TypeString) — not configured, not asserted.\n- if (Optional, TypeString) — not configured, not asserted.\n- ignore_failure (Optional, TypeBool, default false) — only covered indirectly via json; not asserted as a schema attribute.\n- on_failure (Optional, TypeList) — not configured, not asserted.\n- tag (Optional, TypeString) — not configured, not asserted.\n\n## Attributes with poor coverage\n- separator (Required, TypeString)\n - Observed: configured once as \"\\\\s+\" and only verified through json.\n - Gaps: no direct attribute assertion, single value only.\n- preserve_trailing (Optional, TypeBool, default false)\n - Observed: only covered indirectly via json default output.\n - Gaps: no attribute-level assertion, no explicit true case.\n- ignore_missing (Optional, TypeBool, default false)\n - Observed: only

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