Add a rawBody optional parameter

[brn]

Motivation and Context

SSEServerTransport requires nodejs IncomingMessage as a first parameter,
but this IncomingMessage need not to be consumed by other body parser middlewares,
because in body-parser, _readableState will be checked and if you already consumed request body, body-parser will be thrown an error.
If you use server framework like fastify, body parser is installed at an initialization timing.

So I'd like to add a third and forth parameter to handlePostMessage to use an already parsed raw body.

How Has This Been Tested?

Yes, link library to real application.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[jspahrsummers]

Thanks for the PR! Unfortunately, this introduces a fair bit of complexity to the code.

I think a better solution is to exclude the body parsing middleware on the route used for an MCP SSE server. Most frameworks provide a way to do this opt-out—if yours doesn't, it seems like that might a better feature request there, versus trying to reverse the middleware here.