fix: removed dependency on sessionid when initializing transporter

[igoyalsamarth]

THIS PR IS A CLEANER VERSION OF #110

Fixed a case where we expected

(!sessionId && isInitializeRequest(req.body)

to be just

(isInitializeRequest(req.body))

considering that we have to create a new transporter instance, we don't care if the sessionId already exists or not, and the transporter is creating the new sessionId for us anyways.

The old code was also not compatible with

import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp";

and was giving me errors around the sessionId every time it had to initiazlied when coupled with

import { experimental_createMCPClient } from "ai";

(Vercel AI SDK)