Accept passthrough headers in agent execute

[jiapingzeng]

Description

Create MCP client on each execute request + forward headers to MCP connector

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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Accepted headers:

x-amzn-fas-accesskey, x-amzn-fas-secretkey, x-amzn-fas-sessiontoken, x-amzn-datasources

Sample request:

POST {{endpoint}}/_plugins/_ml/agents/{{agent_id}}/_execute
--header 'x-amzn-fas-accesskey: example' \
--header 'x-amzn-fas-secretkey: example' \
--header 'x-amzn-fas-sessiontoken: example' \
--header 'x-amzn-datasources: https://example.aos.us-east-1.on.aws' \
--header 'Content-Type: application/json'
{
    "parameters": {
        "question": "list indices"
    },
    "verbose": "true"
}

Response:

{
    "inference_results": [
        {
            "output": [
                {
                    "name": "memory_id",
                    "result": "F7vALZoBCqjo4z06S5j0"
                },
                {
                    "name": "parent_interaction_id",
                    "result": "GbvALZoBCqjo4z06TZiW"
                },
                {
                    "name": "response",
                    "dataAsMap": {
                        "response": "Here are the indices in your OpenSearch cluster:\n\n## User Indices\n1. **tmdb** - 8,516 documents, green health\n...",
                        "additional_info": {}
                    }
                }
            ]
        }
    ]
}

[Zhangxunmt]

spotless needs to apply

[dhrubo-os]

[nit] Are we planning to extract any other headers than MCP_REQUEST_HEADERS_THREAD_CONTEXT_KEY in this method if future? If not, suggest to rename the name to be more specific to this use case.

[Hailong-am]

can we change it to x-amzn-datasource-endpoint and add a new header x-amzn-datasource-type