Describe the bug
According to the docs (and the relevant portion of the MCP spec), tool execution errors should look like the following:

{
  "jsonrpc": "2.0",
  "id": 4,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Failed to fetch weather data: API rate limit exceeded"
      }
    ],
    "isError": true
  }
}

But instead, they look like the following (i.e., protocol error format):

{
  "jsonrpc": "2.0",
  "id": 4,
  "error": {
    "code": -32603,
    "message": "Internal error",
    "data": "Internal error calling tool fetch_weather_data"
  }
}

So the actual behavior of the MCP gem stack is:

1. Tool raises an exception (e.g., ActiveRecord::RecordNotFound)
2. MCP Server's call_tool catches it and re-raises as RequestHandlerError
3. Seemingly, it is caught and re-raised here and somewhere JsonRpcHandler catches that and returns a JSON-RPC error response (had a bit of trouble finding exactly where the caught error lands).

Unless I'm missing something, this means the MCP gem documentation is incorrect - it does not automatically convert exceptions to isError: true responses. Instead, raising exceptions get converted to JSON-RPC protocol errors (code -32603).

Returning Tool::Response.new(content, error: true) would work as documented, but requires manual exception handling as a workaround.

To Reproduce
Steps to reproduce the behavior:

1. Raise an error inside a tool call.
2. Verify that response matches what the MCP spec deems a protocol error rather than a tool execution error.

Expected behavior
I expect for raised errors within tool calls (e.g., ActiveRecord::RecordNotFound) that are not deemed internal server errors to be handled intuitively, as tool call execution errors, rather than protocol (-32603) errors. This is what the README says should be the current default behavior for raised errors within tool calls.