Add ergonomic test client for testing MCP servers #1728

Summary

Provide a dedicated test client that simplifies testing MCP servers without the boilerplate of manually setting up transports, streams, and context managers.

Motivation

Currently, testing an MCP server requires:

• Setting up stdio_client or similar transport
• Managing multiple nested context managers (transport + session)
• Wiring up memory streams manually

This creates friction for both SDK users writing tests for their servers and for the SDK's own test suite.

Goal

Something analogous to FastAPI's TestClient / Starlette's test client pattern - a simple way to instantiate a client against a server instance and make requests without transport ceremony.

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