[BUG] Custom ISM policy isnt injected

Describe the bug
On fresh start (DataPrepper against fresh empty initialized OpenSearch DB) we expecting, that DataPrepper inject our custom ISM policy + custom index template.

From logs from DataPrepper I can see, that the index template is managed, but ISM policy not.

Expected behavior
If custom ISM policy is used, must be showed in ISM management GUI and ISM must managing the OTel indices.

Environment (please complete the following information):

• DataPrepper Docker image v 2.6.2
• OpenSearch Docker image v2.11.1

Additional context

/usr/share/data-prepper/config/data-prepper-config.yaml

ssl: false

/usr/share/data-prepper/otel-span-index-template.json

{
  "index_patterns": ["otel-v1-apm-span-*"],
  "version": 1,
  "template": {
    "settings": {
      "plugins.index_state_management.rollover_alias": "otel-v1-apm-span"
    }
  }
}

/usr/share/data-prepper/otel-span-ism-policy.json

{
  "policy": {
    "policy_id": "otel-span",
    "description": "Managing raw spans for trace analytics",
    "default_state": "current_write_index",
    "states": [
      {
        "name": "current_write_index",
        "actions": [
            {
                "rollover": {
                    "min_size": "10gb",
                    "min_index_age": "24h"
                }
            }
        ],
        "transitions": [
            {
                "state_name": "delete",
                "conditions": {
                    "min_index_age": "3d"
                }
            }
        ]
      },
      {
        "name": "delete",
        "actions": [
          {
            "delete": {}
          }
        ]
      }
    ],
    "ism_template": [
      {
        "index_patterns": ["otel-v1-apm-span-*"]
      }
    ]
  }
}

/usr/share/data-prepper/pipelines/pipelines.yaml

entry-pipeline:
  workers: 4
  delay: "100"
  source:
    otel_trace_source:
      ssl: true
      sslKeyCertChainFile: "/usr/share/data-prepper/server.crt"
      sslKeyFile: "/usr/share/data-prepper/server.key"
      port: 21890
      authentication:
        http_basic:
          username: REDACTED
          password: REDACTED
  buffer:
    bounded_blocking:
      buffer_size: 1024
      batch_size: 256
  sink:
    - pipeline:
        name: "raw-pipeline"
    - pipeline:
        name: "service-map-pipeline"

raw-pipeline:
  source:
    pipeline:
      name: "entry-pipeline"
  processor:
    - otel_traces:
  sink:
    - opensearch:
        #index_type: trace-analytics-raw
        index_type: custom
        index: otel-v1-apm-span
        hosts: [ https://os-endpoint.REDACTED:9200 ]
        cert: "/usr/share/data-prepper/root-ca.pem"
        template_file: "/usr/share/data-prepper/otel-span-index-template.json"
        ism_policy_file: "/usr/share/data-prepper/otel-span-ism-policy.json"
        username: REDACTED
        password: REDACTED

service-map-pipeline:
  delay: "100"
  source:
    pipeline:
      name: "entry-pipeline"
  processor:
    - service_map:
  sink:
    - opensearch:
        index_type: trace-analytics-service-map
        hosts: [ https://os-endpoint.REDACTED:9200 ]
        cert: "/usr/share/data-prepper/root-ca.pem"
        username: REDACTED
        password: REDACTED

Start by reproducing the fresh-start setup with DataPrepper 2.6.2 and OpenSearch 2.11.1 using pipelines.yaml, the custom otel-span-index-template.json, and otel-span-ism-policy.json. Check the OpenSearch sink's handling of ism_policy_file and its logs, then verify that the custom policy appears in ISM management and manages the OTel indices.