[BUG] RAG: Handle BWC for bedrock converse API

[pyek-bot]

What is the bug?
Support for bedrock converse API was added in 2.18. Along with it, a new field called llmMessages was added to GenerativeQaParameters.java If you upgrade the cluster from 2.17 to 2.18, the byte streams are incompatible for the request and response. This needs to be handled gracefully.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Create a cluster with one master and one data node
2. Create a rag pipeline
3. Send request to the rag pipeline

What is the expected behavior?
Request should be successfully processed and response returned.

Do you have any screenshots?

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_state_exception",
        "reason": "unexpected byte [0x04]"
      }
    ],
    "type": "search_phase_execution_exception",
    "reason": "all shards failed",
    "phase": "query",
    "grouped": true,
    "failed_shards": [
      {
        "shard": 0,
        "index": "my_rag_test_data_1",
        "node": "_IF1uLW5THyB_ZlfxcFRqQ",
        "reason": {
          "type": "illegal_state_exception",
          "reason": "unexpected byte [0x04]"
        }
      }
    ],
    "caused_by": {
      "type": "illegal_state_exception",
      "reason": "unexpected byte [0x04]",
      "caused_by": {
        "type": "illegal_state_exception",
        "reason": "unexpected byte [0x04]"
      }
    }
  },
  "status": 500
}

Example stack trace:

2024-10-09 14:34:47 Caused by: org.opensearch.OpenSearchException$3: unexpected byte [0x02]
2024-10-09 14:34:47     at org.opensearch.OpenSearchException.guessRootCauses(OpenSearchException.java:710) ~[opensearch-core-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     at org.opensearch.action.search.AbstractSearchAsyncAction.executeNextPhase(AbstractSearchAsyncAction.java:393) [opensearch-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     ... 51 more
2024-10-09 14:34:47 Caused by: java.lang.IllegalStateException: unexpected byte [0x02]
2024-10-09 14:34:47     at org.opensearch.core.common.io.stream.StreamInput.readBoolean(StreamInput.java:596) ~[opensearch-core-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     at org.opensearch.core.common.io.stream.StreamInput.readBoolean(StreamInput.java:586) ~[opensearch-core-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     at org.opensearch.search.builder.SearchSourceBuilder.<init>(SearchSourceBuilder.java:302) ~[opensearch-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     at org.opensearch.core.common.io.stream.StreamInput.readOptionalWriteable(StreamInput.java:1013) ~[opensearch-core-2.18.0.jar:2.18.0]
2024-10-09 14:34:47     at org.opensearch.search.internal.ShardSearchRequest.<init>(ShardSearchRequest.java:245) ~[opensearch-2.18.0.jar:2.18.0]
...

Context:
#2851