fix: fix sse deserialization

[SergeyMenshykh]

This PR fixes the deserialization of SSE events by polymorphically deserializing the event data from the result property (see the JSON example below) instead of the event(entire JSON object) itself:

{  
    "id": "4e5f4e5a-9b2c-485b-b6dc-4ddbeb1988d1",  
    "jsonrpc": "2.0",  
    "result": {  
        "kind": "message",  
        "messageId": "36818e79-700c-4795-a2e5-bf7f48f38fb3",  
        "parts": [  
            {  
                "kind": "text",  
                "text": "Hello World"  
            }  
        ],  
        "role": "agent"  
    }  
}  

[gemini-code-assist]

Summary of Changes

Hello @SergeyMenshykh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical bug in the deserialization of Server-Sent Events (SSE) by ensuring that the client correctly parses the event data from the result field of a JSON-RPC response. This change is crucial for the client to properly interpret and process incoming SSE messages, and it is supported by corresponding updates to the client sample application and unit tests to validate the new deserialization logic.

Highlights

• SSE Deserialization Fix: Modified the A2AClient to correctly deserialize Server-Sent Events (SSE) by extracting the event data from the result property of a JSON-RPC response, rather than attempting to deserialize the entire JSON object. This aligns with the expected JSON-RPC envelope for SSE data.
• Client Sample Update: Enhanced the Program.cs sample client to include MessageId and ContextId when sending messages, and added specific handling for Message type events received via SSE, including printing message details and text parts.
• Unit Test Adjustments: Updated several unit tests in A2AClientTests.cs to reflect the new JSON-RPC response structure for SSE events, ensuring that simulated responses now correctly wrap event data within a result property.

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[gemini-code-assist]

Code Review

This pull request fixes an issue with SSE deserialization by correctly parsing the JSON-RPC response structure. The changes in the client logic are sound, and the accompanying updates to the sample application and unit tests are appropriate. I've suggested an improvement to the error handling in the SSE deserialization logic to make it more robust by explicitly handling JSON-RPC error responses.