CallToolResult deserialization fails when content block contains unexpected properties

[jeffhandley]

Describe the bug
When a tool call result contains unexpected properties in the content, the deserializer attempts to process the data. When that property is an array or object, it puts the converter into a bad state, leading to an exception.

To Reproduce

using ModelContextProtocol;
using ModelContextProtocol.Protocol;
using System.Text.Json;

const string malformedCallToolResultJson = """
{
    "content": [{
        "type": "text",
        "text": "[\n  {\n    \"Data\": \"1234567890\"\n  }\n]",
        "structuredContent": [ { "Data": "1234567890" } ]
    }]
}
""";

// Try to deserialize the unexpected CallToolResult using McpJsonUtilities.DefaultOptions
// This currently throws because the ContentBlock.Converter doesn't properly skip unknown properties
// that contain complex structures like arrays or objects
try
{
    JsonSerializer.Deserialize<CallToolResult>(malformedCallToolResultJson, McpJsonUtilities.DefaultOptions);
}
catch (JsonException ex)
{
    Console.WriteLine("Caught JsonException as expected:");
    Console.WriteLine(ex.Message);
}

This currently throws:

Caught JsonException as expected:
The converter 'ModelContextProtocol.Protocol.ContentBlock+Converter' read too much or not enough. Path: $.content[0] | LineNumber: 6 | BytePositionInLine: 1.

Expected behavior
Deserialization should succeed without an exception being thrown.