fix(tests): replace sleep with anyio.Event in test_session

Replace `await anyio.sleep(0.1)` in `run_server` with an `anyio.Event`
that the mock client sets after receiving the error response. The server
now waits on a precise signal instead of guessing how long the client
needs.

Author: Kludex

tests/server/test_session.py

@@ -449,6 +449,7 @@ async def test_other_requests_blocked_before_initialization():
 
     error_response_received = False
     error_code = None
+    client_done = anyio.Event()
 
     async def run_server():
         async with ServerSession(
@@ -460,9 +461,8 @@ async def run_server():
                 capabilities=ServerCapabilities(),
             ),
         ):
-            # Server should handle the request and send an error response
-            # No need to process incoming_messages since the error is handled automatically
-            await anyio.sleep(0.1)  # Give time for the request to be processed
+            # Wait until the client has received and verified the error response
+            await client_done.wait()
 
     async def mock_client():
         nonlocal error_response_received, error_code
@@ -478,6 +478,8 @@ async def mock_client():
             error_response_received = True
             error_code = error_message.message.error.code
 
+        client_done.set()
+
     async with (
         client_to_server_send,
         client_to_server_receive,