fix(os-persistence): Migrate index templates to Composable API with priorities

[Akhil-Pathivada]

Pull Request type

• Bugfix

Changes in this PR

This PR fixes a critical bug where OpenSearch index templates were not applying field mappings correctly, resulting in indices being created with empty mappings for task_log, event, and message indices.

Root Cause

The code was using the deprecated Legacy Template API (PUT /_template/) while the template JSON files were structured in the Composable Template API format (with nested "template": {} key). The Legacy API does not recognize the nested template structure and silently ignored it, resulting in templates being created with empty mappings.

Solution

1. Migrated to Composable Template API: Updated OpenSearchRestDAO to use /_index_template/ endpoint instead of /_template/
2. Added template priorities: OpenSearch's Composable API requires different priorities when index patterns could potentially overlap. Added priorities (1, 2, 3) to satisfy this validation requirement
3. Enhanced log field for full-text search: Changed log field type from keyword to text in template_task_log.json to enable full-text search capabilities on task execution logs
4. Updated test assertions: Modified TestOpenSearchRestDAO to verify templates using the new API endpoint

Why This Approach

• Uses recommended Composable Template API (Legacy API is deprecated)
• Priorities satisfy OpenSearch validation for overlapping patterns
• Text field enables full-text log search vs exact-match-only keyword type

Testing

• Verified templates are created automatically on startup & proper field mappings are applied
• All three templates (task_log, event, message) creating successfully with correct field types

Alternatives considered

1. Legacy API format: Convert JSON templates to flat structure for Legacy API. Rejected because /_template/ is deprecated in OpenSearch.

2. Specific patterns without priorities: Use patterns like *_task_log_* to avoid overlap. Rejected because OpenSearch validation still requires priorities for any potential pattern conflicts.

[Akhil-Pathivada]

@v1r3n @manan164 can you please help in reviewing this?