Provide an option to increase request payload size in server implementation

[siwachabhi]

Is your feature request related to a problem? Please describe.
When building a server which can accept larger than 4MB payload size, current implementation has a fix limit to reject it.

Describe the solution you'd like
Make this a configurable parameter, so servers can configure

Describe alternatives you've considered
Workaround is to use something like this: https://stackoverflow.com/questions/12496089/how-can-i-override-a-constant-in-an-imported-python-module, which can backward incompatible.

Other options is to provide a separate path for larger file I/O, but that seems redundant.

Additional context
https://github.com/modelcontextprotocol/python-sdk/blob/544176770b53e6a0ae8c413d3b6c5116421f67df/src/mcp/server/streamable_http.py#L337-L344

[siwachabhi]

Wrong repo