SDK Parity: Avoid Parsing Server Response for non-JsonRPCMessage Requests

[justin-yi-wang]

Achieve parity with the TypeScript SDK by only parsing server responses to JsonRPCRequests.

Motivation and Context

In the TypeScript SDK, server responses are only parsed if the request is a JsonRPCRequest (ie. has "method" and "id" fields) - link. Otherwise, the response is ignored. Here in the Python SDK, all responses are parsed. Concretely, we've seen in the wild that servers may try to respond to notifications with a 204 No content rather than a 202 Accepted (which is special cased in both SDKs), resulting in a Pydantic validation error. Subsequently, server developers are confused why their servers work with clients presumably using the TypeScript SDK, but not ones using the Python SDK.

How Has This Been Tested?

This PR includes a test.

Breaking Changes

No. Per the MCP protocol, the server should not respond to notifications.

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

[ochafik]

Thanks @justin-yi-wang!

[ochafik]

Thanks again, two last nits!

[ochafik]

Thanks!