fix(session): return INVALID_REQUEST error instead of crashing on pre-init requests

[Smeet23]

Problem

When an MCP client sends a request (e.g. tools/list) before completing the initialize handshake, the server raises a RuntimeError that propagates through the anyio task group and crashes the entire ASGI application:

RuntimeError: Received request before initialization was complete
ExceptionGroup: unhandled errors in a TaskGroup
ERROR: Exception in ASGI application

This affects real-world clients such as Cursor, MCP Inspector, and anything-llm that skip re-initialization after a server restart or dropped SSE connection. The server dies instead of returning a useful error.

Fixes #423.

Solution

Replace the raise RuntimeError(...) in ServerSession._received_request with a proper JSON-RPC INVALID_REQUEST (-32600) error response sent via the responder. The server stays alive and the client receives a meaningful error it can act on.

# Before
case _:
    if self._initialization_state != InitializationState.Initialized:
        raise RuntimeError("Received request before initialization was complete")

# After
case _:
    if self._initialization_state != InitializationState.Initialized:
        with responder:
            await responder.respond(
                ErrorData(
                    code=INVALID_REQUEST,
                    message="Received request before initialization was complete",
                )
            )
        return

Test

Added test_request_before_initialization_returns_error to tests/server/test_session.py that sends a tools/list request without any prior initialize handshake and asserts that:

• The server returns a JSONRPCError with code INVALID_REQUEST
• The server does not crash (no ExceptionGroup)