[FIX] error is not allowed as content type

[weese]

mcp-framework uses content type error to represent errors which is not compliant with the MCP standard as defined here:
https://modelcontextprotocol.io/specification/2025-06-18/server/tools

It breaks the standard clients e.g. from https://modelcontextprotocol.io/ that cannot parse and fail with unsupported content type: error.

I fixed it in my PR and successfully tested it.

Current and invalid error response:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [
      {
        "type": "error",
        "text": "Invalid number"
      }
    ]
  }
}

Expected error response as defined in the standard:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Invalid number"
      }
    ],
    "isError": true
  }
}