[v1.x] test(stdio): regression test for #1960 BrokenResourceError race

Covers the fix in the previous commit. Spawns a subprocess that emits a
burst of valid JSONRPC notifications, exits the stdio_client context
immediately, asserts no exception propagates. Fails before the fix
(ExceptionGroup / BrokenResourceError), passes after. Wrapped in
anyio.fail_after(5.0) per AGENTS.md.

Github-Issue:#1960

Author: BashNetCorp

tests/client/test_stdio.py

@@ -33,6 +33,45 @@ async def test_stdio_context_manager_exiting():
         pass
 
 
+@pytest.mark.anyio
+async def test_stdio_client_exits_cleanly_while_server_still_writing():
+    """Regression test for #1960.
+
+    Exiting the ``stdio_client`` context while the subprocess is still writing to
+    stdout used to surface ``anyio.BrokenResourceError`` through the task group
+    (as an ``ExceptionGroup``). The ``finally`` block closes
+    ``read_stream_writer`` while the background ``stdout_reader`` task is
+    mid-``send``.
+
+    The fix makes ``stdout_reader`` catch both ``ClosedResourceError`` and
+    ``BrokenResourceError`` and return cleanly, so exiting the context is a
+    no-op no matter what the subprocess is doing.
+    """
+    # A server that emits a large burst of valid JSON-RPC notifications without
+    # ever reading stdin. When we exit the context below, the subprocess is
+    # still in the middle of that burst, which is the exact shape of the race.
+    noisy_script = textwrap.dedent(
+        """
+        import sys
+        for i in range(1000):
+            sys.stdout.write(
+                '{"jsonrpc":"2.0","method":"notifications/message",'
+                '"params":{"level":"info","data":"line ' + str(i) + '"}}\\n'
+            )
+            sys.stdout.flush()
+        """
+    )
+
+    server_params = StdioServerParameters(command=sys.executable, args=["-c", noisy_script])
+
+    # The ``async with`` must complete without an ``ExceptionGroup`` /
+    # ``BrokenResourceError`` propagating. ``anyio.fail_after`` prevents a
+    # regression from hanging CI.
+    with anyio.fail_after(5.0):
+        async with stdio_client(server_params) as (_, _):
+            pass
+
+
 @pytest.mark.anyio
 @pytest.mark.skipif(tee is None, reason="could not find tee command")
 async def test_stdio_client():