test: gh-106749 heal at test_streamable_http resumption-test cancel site

The test cancels a task group mid-body (line 1381) to kill the first
client session while a tool is blocked; that cancel is the throw, and
the second session's lines (1399-1404) fall in the desynced window.
Same checkpoint pattern, applied at the actual site (the autouse
fixture is a cross-test backstop and cannot help an intra-test
desync).

Author: maxisbey

tests/shared/test_streamable_http.py

@@ -18,6 +18,7 @@
 from urllib.parse import urlparse
 
 import anyio
+import anyio.lowlevel
 import httpx
 import pytest
 import requests
@@ -1379,6 +1380,11 @@ async def run_tool():
                     # Kill the client session while tool is waiting on lock
                     tg.cancel_scope.cancel()
 
+    # gh-106749 heal: the cancel above is delivered via coro.throw() on
+    # CPython 3.11; resume via .send() so coverage's CTracer re-syncs before
+    # the second session's lines.
+    await anyio.lowlevel.cancel_shielded_checkpoint()
+
     async with create_mcp_http_client(headers=headers) as httpx_client2:
         async with streamable_http_client(f"{server_url}/mcp", http_client=httpx_client2) as (
             read_stream,