Calling a nonexistent tool should trigger a JSON-RPC Error

[chemicL]

Describe the bug
When a client attempts to call a tool that does not exist the specification requires that a protocol error is raised as a JSON-RPC Error. Instead, the current SDK returns a JSON-RPC Result with isError = true.

To Reproduce
Steps to reproduce the behavior:

1. Call a missing tool

Expected behavior
As per the specification, the result should be:

{
  "jsonrpc": "2.0",
  "id": 3,
  "error": {
    "code": -32602,
    "message": "Unknown tool: nonexistent_tool"
  }
}

while actually it's

{
    "jsonrpc": "2.0",
    "id": "3",
    "result": {
      "content": [
        {
          "type": "text",
          "text": "MCP error -32602: Tool nonexistent_tool not found"
        }
      ],
      "isError": true
    }
  }

Additional context
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/packages/server/src/server/mcp.ts#L201-L204
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/packages/server/src/server/mcp.ts#L211-L227
are at fault.

[chemicL]

I believe this behaviour is intended looking at #1044. However, it is against the specification. Perhaps the spec has to change?